Add build-result job to collect matrix results (#500)

The `build` matrix runs on all three platforms and passes reliably, so all
of them can be required. Instead of listing each platform in the ruleset,
add a single `build-result` job that fails unless the whole matrix
succeeded, and require that check in `.asf.yaml`.

Assisted-By: Claude Fable 5.1 <noreply@anthropic.com>

Author: ppkarwasz

.asf.yaml

@@ -75,11 +75,7 @@ github:
       required_status_checks:
         # The GitHub Actions app
         - app_slug: github-actions
-          name: "build / build (macos-latest)"
-        - app_slug: github-actions
-          name: "build / build (ubuntu-latest)"
-        - app_slug: github-actions
-          name: "build / build (windows-latest)"
+          name: "build-result"
         # The GitHub Advanced Security app
         - app_slug: github-advanced-security
           name: "CodeQL"

.github/workflows/build.yaml

@@ -46,6 +46,22 @@ jobs:
       site-enabled: true
       reproducibility-check-enabled: false
 
+  # Collects the results of the `build` matrix into a single check.
+  # This is the only check that needs to be listed as required in `.asf.yaml`.
+  build-result:
+    needs: build
+    # Run even if `build` failed or was canceled, so that the check is always reported.
+    if: always()
+    runs-on: ubuntu-slim
+    steps:
+      - name: Verify build result
+        env:
+          # `success` only if all the jobs of the matrix succeeded.
+          BUILD_RESULT: ${{ needs.build.result }}
+        run: |
+          echo "Build result: $BUILD_RESULT"
+          test "$BUILD_RESULT" = "success"
+
   deploy-snapshot:
     needs: build
     if: github.repository == 'apache/logging-flume' && github.ref_name == 'trunk'