Add a success job to CI

This will allow us to enable auto merge once CI completes.

library/compiler-builtins/libm/.github/workflows/main.yml

@@ -71,3 +71,20 @@ jobs:
     - name: Install Rust
       run: rustup update nightly && rustup default nightly
     - run: cargo bench --all
+
+  success:
+    needs:
+      - docker
+      - rustfmt
+      - wasm
+      - cb
+      - benchmarks
+    runs-on: ubuntu-latest
+    # 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) }}'