GitHub Actions framework cache not working as expected

[lobis]

When debugging the following PR: #365 I found an error in the GitHub Actions workflow.

It seems that the caching of the framework is instead restoring the cache (because a key already exists) but this should be unique each workflow run, so this should not happen. One can see this in the following run:

https://github.com/rest-for-physics/framework/actions/runs/4913830534/jobs/8774448343

In particular the workflow is failing because the installation directory for Garfield differs between Docker images, the installation with the correct image is not being saved into the cache because the key is already populated.

I guess we should use a more robust cache key identifier so this cannot happen.