Add POOCH_BASE_URL to specify the base url used by pooch to download test data

[ericpre]

This will fix the package_and_test.yml workflow after adding some files in a PR (see for example #162 (comment)) and also the release.yml workflow when running from a fork for testing the workflow (it is current working fine for a "real" run because the tag is then available in the hyperspy/rosettasciio repository).
I though it would be more complicated to fix and possibly not worth making something complicated but this solution is actually very simple.

Progress of the PR

• Add POOCH_BASE_URL to specify the base url used by pooch to download test data,
• [n/a] update docstring (if appropriate),
• update user guide (if appropriate),
• add a changelog entry in the upcoming_changes folder (see upcoming_changes/README.rst),
• Check formatting of the changelog entry (and eventual user guide changes) in the docs/readthedocs.org:rosettasciio build of this PR (link in github checks)
• add tests,
• ready for review.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (d0052ce) 86.25% compared to head (9e936c9) 86.23%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #200      +/-   ##
==========================================
- Coverage   86.25%   86.23%   -0.02%     
==========================================
  Files          82       82              
  Lines       10541    10544       +3     
  Branches     2291     2291              
==========================================
+ Hits         9092     9093       +1     
- Misses        931      933       +2     
  Partials      518      518              

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[ericpre]

For reference, I had to fiddle with finding the correct github context variable for push and pull_request event, which is not very from the github documentation but actually event payload can viewed as documented in https://docs.github.com/en/webhooks/testing-and-troubleshooting-webhooks/viewing-webhook-deliveries.