The Code Climate action apparently needs to run in a repo (#32)

The Code Climate action, used to upload coverage reports appears to
need to run in a git repo. Add a checkout action to check out the
repo.

.github/workflows/upload_coverage.yml

@@ -12,6 +12,8 @@ jobs:
  if: github.event.workflow_run.conclusion == 'success'
 
  steps:
+ - uses: actions/checkout@v4
+
  - name: Fetch coverage report
  uses: actions/download-artifact@v4
  with: