fix(appstream): isolate CI env vars in resolver tests

[abn]

Root Cause

During tag/release workflow runs, CI environment variables like GITHUB_REF (refs/tags/v0.33.0), GITHUB_REF_TYPE, and GITHUB_REF_NAME are populated in the runner's ambient environment. Because pkg/appstream/resolver_test.go did not isolate CI environment variables, TestResolveRelease_GitMock matched the ambient CI environment variable branch before reaching the mocked Git command branch, failing the assertions.

Solution

1. Added environment variable isolation (clearCIEnv) using t.Setenv in pkg/appstream/resolver_test.go for all tests.
2. Added comprehensive subtests for all CI environments and Git tag fallback cases.
3. Updated pkg/appstream/resolver.go to retain rawGitTag when matching CI tag environment variables so commit dates can also be extracted via Git log when available.