Disable coveralls job if the needed secrets aren’t available

For security, GitHub Actions does not make secrets available to pull
requests from external contributors, so the coveralls job was always
failing for those PRs.  Disable this job when secrets are not
available.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

Author: andersk

.github/workflows/python-package.yml

@@ -70,6 +70,7 @@ jobs:
         make flake8
         make black
   coveralls:
+    if: ${{ github.secret_source == 'Actions' }}
     runs-on: ubuntu-22.04
     environment: CI
     steps: