Check for code changes in github actions

[guarin]

We would like to make unit tests required before merging. This is currently not possible as not all code changes trigger the unit test action (see here). This results in github waiting for an action to pass that is never scheduled.

To fix this we should use https://github.com/dorny/paths-filter instead of relying on workflow ignore filters for the unit test actions. This would trigger the action and make it complete quickly if no code changes are detected.