[ci] Add omnibus 'success' job. (google#443)

This job is used to signal to branch protections that all other
jobs have succeeded.

.github/workflows/ci.yml

@@ -441,3 +441,13 @@ jobs:
           cargo install cargo-readme --version 3.2.0 &> /dev/null || true
           cargo install --locked kani-verifier       &> /dev/null || true
           cargo kani setup                           &> /dev/null || true
+
+  # Used to signal to branch protections that all other jobs have succeeded.
+  all-jobs-succeed:
+      name: All checks succeeded
+      if: success()
+      runs-on: ubuntu-latest
+      needs: [build_test, kani, check_fmt, check_readme, check_msrv, check_versions, generate_cache]
+      steps:
+        - name: Mark the job as successful
+          run: exit 0