CI: Multiple tweaks to the CI workflow file

[DilumAluthge]

1. Update actions/checkout from v2 to v4.
2. Use Dependabot to keep GitHub Actions up-to-date.
3. Add a timeout to the test job.
4. Set contents: read for the GITHUB_TOKEN (because we don't need any write permissions).
5. Set persist-credentials: false in actions/checkout.
6. Pass the matrix value through an intermediate environment variable.
7. Disable fast-failing.

[DilumAluthge]

Alright, this one is also ready for review @kleinhenz.