ci: consider making path ignoring more robust

I'm creating an issue to track what I mentioned in [this comment](https://github.com/crashappsec/chalk/pull/227/files/d2fef48de0c022d67ef05c7f23c4ef2a107bf148#r1509448563), since the failure mode just happened.

PR https://github.com/crashappsec/chalk/pull/254 passed checks at PR time and on `main`, but only because the tests workflow was skipped (the `Dockerfile` was not included in the list of paths for which the workflow should run). It's easy to miss that a workflow was skipped when creating or reviewing a PR, so it then caused CI failures in subsequent PRs.

I argue that it is difficult to maintain a list of paths on which a workflow should run, and more robust to maintain a list of paths on which the workflow should not run.

It is better for a workflow to run on a PR when it doesn't need to, than not run on a PR when it should.