Replace the Node.js action with a composite action running a Bash script

.github/workflows/parity.yml

@@ -0,0 +1,96 @@
+name: Test Bash implementation
+
+# Tests install.sh on its own and against the Node implementation
+# it replaced. End-to-end tests of the action are in test.yml.
+
+permissions: read-all
+
+on:
+  workflow_dispatch:
+  pull_request:
+
+jobs:
+  lint:
+    runs-on: ubuntu-24.04
+    steps:
+      - name: Harden Runner
+        uses: step-security/harden-runner@17d0e2bd7d51742c71671bd19fa12bdc9d40a3d6 # v2.8.1
+        with:
+          egress-policy: audit
+
+      - name: Checkout repository
+        uses: actions/checkout@692973e3d937129bcbf40652eb9f2f61becf3332 # v4.1.7
+
+      - name: Shellcheck
+        run: shellcheck install.sh test/*.sh
+
+  unit:
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+          - os: ubuntu-24.04
+            bash: bash
+          # macOS /bin/bash is 3.2, which self-hosted macOS runners may still use.
+          - os: macos-15
+            bash: /bin/bash
+          - os: macos-15
+            bash: bash
+    runs-on: ${{ matrix.os }}
+    steps:
+      - name: Harden Runner
+        uses: step-security/harden-runner@17d0e2bd7d51742c71671bd19fa12bdc9d40a3d6 # v2.8.1
+        with:
+          egress-policy: audit
+
+      - name: Checkout repository
+        uses: actions/checkout@692973e3d937129bcbf40652eb9f2f61becf3332 # v4.1.7
+
+      - name: Unit tests
+        run: ${{ matrix.bash }} test/unit.sh
+
+      - name: Real download timeout tests
+        if: runner.os == 'Linux'
+        run: python3 test/download-timeout.py
+
+  windows-zip:
+    runs-on: windows-2022
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@692973e3d937129bcbf40652eb9f2f61becf3332 # v4.1.7
+
+      - name: ZIP extraction without unzip
+        shell: bash
+        run: bash test/windows-zip.sh
+
+  parity:
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+          # Explicit platform/arch cases don't depend on the host, so they run once.
+          - os: ubuntu-24.04
+            mode: full
+          - os: ubuntu-24.04-arm
+            mode: host
+          - os: macos-15
+            mode: host
+          - os: macos-15-intel
+            mode: host
+          - os: windows-2022
+            mode: host
+          - os: windows-11-arm
+            mode: host
+    runs-on: ${{ matrix.os }}
+    steps:
+      - name: Harden Runner
+        uses: step-security/harden-runner@17d0e2bd7d51742c71671bd19fa12bdc9d40a3d6 # v2.8.1
+        with:
+          egress-policy: audit
+
+      - name: Checkout repository
+        uses: actions/checkout@692973e3d937129bcbf40652eb9f2f61becf3332 # v4.1.7
+
+      - name: Old vs new
+        shell: bash
+        run: bash test/parity.sh ${{ matrix.mode }}