chore: adopt conclusion job for stable status check (#700)

Rolls out bazel-contrib/rules-template#124

Author: alexeagle

.github/workflows/ci.yaml

@@ -198,3 +198,24 @@ jobs:
               run: |
                   echo "import $GITHUB_WORKSPACE/.github/workflows/ci.bazelrc" > .bazelrc
                   bats -r .
+
+    # For branch protection settings, this job provides a "stable" name that can be used to gate PR merges
+    # on "all matrix jobs were successful".
+    conclusion:
+        needs: test
+        runs-on: ubuntu-latest
+        if: always()
+        steps:
+            - uses: technote-space/workflow-conclusion-action@45ce8e0eb155657ab8ccf346ade734257fd196a5 # v3.0.3
+
+            # Note: possible conclusion values:
+            # https://github.com/technote-space/workflow-conclusion-action/blob/main/src/constant.ts
+            - name: report success
+              if: ${{ env.WORKFLOW_CONCLUSION == 'success' }}
+              working-directory: /tmp
+              run: echo ${{ env.WORKFLOW_CONCLUSION }} && exit 0
+
+            - name: report failure
+              if: ${{ env.WORKFLOW_CONCLUSION == 'failure' }}
+              working-directory: /tmp
+              run: echo ${{ env.WORKFLOW_CONCLUSION }} && exit 1