Run end-to-end tests only if corresponding files have been modified

[paescuj]

This ensures that the end-to-end tests are only performed if files have been modified that match the following patterns:

• api/**/*
• tests/**/*
• docker-compose.yml
• .github/workflows/e2e-tests.yml

[paescuj]

FYI @rijkvanzanten: Similar to the e2e tests we could configure other filter rules to run individual jobs only if the changed files match the defined path patterns...
Thanks to the paths_filter option (I'm going to update the description of the PR soon 😄) in the skip-duplicate-actions.

[paescuj]

Sorry @rijkvanzanten, one last note on this: You might want to add the "End-to-End Tests / Result" as a required check on pull requests, since a) the tests now run only if appropriate and b) they generally seem to work now.

(If you're interested in the possibilities that arise with the new feature in 'skip-duplicate-actions', I've now updated the description of the pull request...)