Cache atlases in CI for brainmapper tests

[sfmig]

Description

What is this PR

• Bug fix
• Addition of a new feature
• Other

Why is this PR needed?
To try and speed up the CI checks, we want to cache the download of atlases for the brainmapper tests.

What does this PR do?
Adds a step in the tests job to cache the .brainglobe folder.

References

#65

How has this PR been tested?

Checks in this PR should pass.

Is this a breaking change?

No.

Does this PR require an update to the documentation?

No.

Checklist:

• [ n/a] The code has been tested locally
• [ n/a] Tests have been added to cover all new functionality (unit & integration)
• [ n/a] The documentation has been updated to reflect any changes
• The code has been formatted with pre-commit

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.45%. Comparing base (b5f62ef) to head (572be3c).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #101      +/-   ##
==========================================
+ Coverage   79.38%   84.45%   +5.06%     
==========================================
  Files          18       17       -1     
  Lines         917      862      -55     
==========================================
  Hits          728      728              
+ Misses        189      134      -55     

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