CI: run macOS jobs on macos-15 until actions/runner-images#14940 is fixed

[ulysses4ever]

Works around #12465; upstream bug: actions/runner-images#14940.

Every PR's macOS jobs have failed since about 2026-10-15 10:30 UTC, regardless of content; master was last green on 2026-10-14 and the merge queue runs the same workflows, so nothing can merge. The macos-26-arm64 image that macos-latest maps to (20261013.0424.1) ships the Command Line Tools 27.0 with the macOS 27 SDK, but keeps Xcode 26.6 as the default toolchain. GHC's link line passes -L/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/lib, so Xcode 26.6's ld reads the SDK 27 library stubs, which list a new arm64e.x1 architecture it cannot parse:

ld: multiple errors: tapi error: malformed file
/Library/Developer/CommandLineTools/SDKs/MacOSX27.0.sdk/usr/lib/libiconv.2.tbd:4:20: error:
     error: unknown architecture
                       arm64e.x1-macos, arm64e.x1-maccatalyst ]

This PR:

• runs the macOS jobs on macos-15 (arm64, with a consistent toolchain: Command Line Tools and Xcode 16.4) instead of macos-latest: the Validate and Dogfooding matrices in validate.yml, the macOS Bootstrap job, and the aarch64 build and test jobs in reusable-release.yml, which the nightly "Build and release" run uses. Each pin carries a comment pointing at macos-26-arm64 20261013.0424.1 broke GHC (linking phase) actions/runner-images#14940, so they can be reverted once the image is fixed; Bootstrap job fails on macos-latest (GHC 9.12.4) due to linker error with SDK TBD files #12465 can stay open to track that.
• sets fail-fast: false on the Bootstrap matrix: the macOS failure was cancelling the four Linux Bootstrap jobs, hiding their results.

Notes for review:

• Check names change from … macos-latest … to … macos-15 …. Only the post jobs are required checks, so branch protection and mergify are unaffected.
• The release-build pin can be dropped from this PR if you'd rather handle releases separately; without it, the macOS aarch64 build in the nightly and release runs hits the same error.
• The 3.18 branch has the same macos-latest lines, so this needs a backport.
• An alternative that keeps macOS 26 is selecting the Command Line Tools as the active toolchain (sudo xcode-select -s /Library/Developer/CommandLineTools, suggested in Bootstrap job fails on macos-latest (GHC 9.12.4) due to linker error with SDK TBD files #12465), so that linker and SDK match again. Nobody has shown it working yet, and it would move all macOS CI onto a new linker at once, so the pin seems the safer unblock.

---

Template B: This PR does not modify behaviour or interface

• Patches conform to the coding conventions.
• Is this a PR that fixes CI? If so, it will need to be backported to older cabal release branches (ask maintainers for directions). — Yes: 3.18 has the same lines.

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[ulysses4ever]

To be clear about scope: this is the most obvious stop-gap, not a fix. The bug is in the runner image (actions/runner-images#14940), and these pins should go once GitHub fixes it. Alternatives that keep us on macOS 26, such as selecting the Command Line Tools as the active toolchain (#12465 (comment)), may well work, but they need investigation before we rely on them.

Why a wholesale pin for now:

• Known-good, self-consistent toolchain. The macos-15 image our jobs run on (20260907.0337.1) has the Command Line Tools and Xcode both at 16.4, so nothing is mixed. The xcode-select route instead puts a brand-new linker, from the 27.0 Command Line Tools, under all six GHCs at once, and we don't know yet how GHC 9.4–9.14 get on with it.
• Already verified. On this PR, Bootstrap passes on macos-15, and all six macOS Validate jobs are past the build steps that used to fail within minutes and are running the test suites.
• No side effects, trivial to revert. A runner label changes nothing at run time. sudo xcode-select changes the developer directory for everything after it in the job, and if left behind after the image is fixed, it would quietly keep us on a toolchain other than the image default. Here, reverting is one word per job, each marked with a comment pointing at the upstream issue.
• Release binaries. For the release builds, a consistent toolchain matters more than OS coverage; I'd rather not ship binaries linked through a workaround.
• Nothing to lose right now. The pin trades macOS 26 coverage, which we don't currently have at all since every macOS job fails, for macOS 15 coverage that works.

One caveat: the macos-15 image hasn't had the October update yet. If GitHub ships it the same Command Line Tools update, this pin stops helping, which is one more reason to look into the xcode-select route in parallel, e.g. in a separate PR across the full matrix.

[mergify]

Merge Queue Status

• ✅ Entered queue — 2026-10-16 19:13 UTC · Rule: default · triggered by rule Put pull requests in the rebase+merge queue
• ✅ Checks skipped · PR is already up-to-date
• ✅ Merged — 2026-10-16 19:23 UTC · at 27789667bb34aa32237289ff5bc861361e431fda · merge

This pull request spent 10 minutes 42 seconds in the queue, including 2 seconds running CI.

Required conditions to merge

• #review-threads-unresolved = 0 [🛡 GitHub branch protection]
  • CI: run macOS jobs on macos-15 until actions/runner-images#14940 is fixed #12468
• github-review-approved [🛡 GitHub branch protection]
  • CI: run macOS jobs on macos-15 until actions/runner-images#14940 is fixed #12468
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