Caching and other improvements for test.yml

[cookpa]

Fixes:

1. Cache data based on the files antspynet/utilities/get_antsxnet_data.py and antspynet/utilities/get_pretrained_network.py.

These are what actually determine what gets downloaded. Caching on the hash of download_all_data.py will result in cache hits
even if data changes.

2. Only update cache once per job. Don't do it in the build matrix. Avoids race conditions and duplicating cache jobs.

3. Specify branches in run conditions, avoids running twice on PRs

4. Build from pyproject.toml, not modified requirements.txt. This allows testing on recent python. Maybe need a separate
   test for requirements.txt install, if it's still needed

We are very close to the 10Gb limit for Github caches - any bigger and jobs might start to fail.

[cookpa]

I hope this is finally working. I had to switch to artifacts instead of a cache. Not ideal, but the runners don't have enough disk space to cache all the data + models