CI: Only run tests on exercises, when exercise relevant files are changed

[vaeng]

I would like to reduce the friction in PRs that run all the exercise tests, although they do not interact with those files.

The bash track shows one way of doing so:

https://github.com/exercism/bash/blob/main/.github/workflows/pr.yml
https://github.com/exercism/bash/blob/main/.github/scripts/pr