ci: key the baseline Git cache to the OS release

.github/actions/prepare-git-compatibility/action.yml

@@ -4,13 +4,25 @@ description: Restore or build the pinned Linux Git binary for the compatibility
 runs:
   using: composite
   steps:
+    # Same scope as install-node-dependencies: a binary linked on a newer image's glibc fails on an older one.
+    - name: Resolve Git cache scope
+      id: git-cache-scope
+      shell: bash
+      run: |
+        scope="${ImageOS:-$RUNNER_OS}"
+        if [ -r /etc/os-release ]; then
+          . /etc/os-release
+          scope="${ID:-linux}-${VERSION_ID:-unknown}"
+        fi
+        echo "scope=$scope" >> "$GITHUB_OUTPUT"
+
     # The default-branch warmer shares this key across PRs; a PR save is private to its merge ref.
     # Cache eviction remains possible, so keep the checksum-verified cold build below.
     - name: Cache baseline Git build
       uses: actions/cache@v5
       with:
         path: ~/.cache/orca-git-compat/git-2.25.5
-        key: git-compat-baseline-${{ runner.os }}-${{ runner.arch }}-2.25.5
+        key: git-compat-baseline-${{ runner.os }}-${{ steps.git-cache-scope.outputs.scope }}-${{ runner.arch }}-2.25.5
 
     # Finish the CPU-heavy build before any timed compatibility lanes start.
     - name: Build the baseline Git binary

config/scripts/git-binary-compatibility-workflow.test.mjs

@@ -60,8 +60,14 @@ describe('Git binary compatibility PR gate', () => {
     // in whichever boundary case is running rather than as a slow build.
     expect(steps[matrixIndex].run).not.toContain('make -C')
     expect(baselineSteps[cacheIndex].with.path).toBe(BASELINE_DIR)
+    // Why the OS release in the key: a baseline linked against a newer image's glibc
+    // fails to load on an older runner while ubuntu-latest rolls over gradually.
+    const scopeIndex = names.indexOf('Resolve Git cache scope')
+    expect(scopeIndex).toBeGreaterThanOrEqual(0)
+    expect(scopeIndex).toBeLessThan(cacheIndex)
+    expect(baselineSteps[scopeIndex].run).toContain('. /etc/os-release')
     expect(baselineSteps[cacheIndex].with.key).toBe(
-      'git-compat-baseline-${{ runner.os }}-${{ runner.arch }}-2.25.5'
+      'git-compat-baseline-${{ runner.os }}-${{ steps.git-cache-scope.outputs.scope }}-${{ runner.arch }}-2.25.5'
     )
   })