test: fail aggregate check if any unit test shard fails

This ensures that the `success` job in unit-tests.yml
always runs, allowing us to protect master from unit
test failures by making the `success` job required.

From https://github.com/marketplace/actions/alls-green#options:

> Important: For this to work properly, it is a must to have the job always run,
> otherwise GitHub will make it skipped when any of the dependencies fail. In
> some contexts, skipped is interpreted as success which may lead to undersired,
> unobvious and even dangerous (as in security breach "dangerous") side-effects.

Closes #34789

.github/workflows/unit-tests.yml

@@ -132,6 +132,7 @@ jobs:
   # https://github.com/orgs/community/discussions/33579
   success:
     name: Unit tests successful
+    if: always()
     runs-on: ubuntu-20.04
     needs: [ run-tests ]
     steps:
@@ -202,3 +203,4 @@ jobs:
           coverage report
           coverage xml
       - uses: codecov/codecov-action@v4
+f