Add a "bors" summary status

.github/workflows/ci.yml

@@ -35,3 +35,18 @@ jobs:
         env:
           TARGET: ${{ matrix.target }}
         run: ./ci/script.sh
+
+  # One job that "summarizes" the success state of this pipeline. This can then be added to branch
+  # protection, rather than having to add each job separately.
+  success:
+    name: bors
+    runs-on: ubuntu-latest
+    needs: [tests]
+    # Github branch protection is exceedingly silly and treats "jobs skipped because a dependency
+    # failed" as success. So we have to do some contortions to ensure the job fails if any of its
+    # dependencies fails.
+    if: always() # make sure this is never "skipped"
+    steps:
+      # Manually check the status of all dependencies. `if: failure()` does not work.
+      - name: check if any dependency failed
+        run: jq --exit-status 'all(.result == "success")' <<< '${{ toJson(needs) }}'