Run github actiosns CI tests on Windows, Mac and Linux

[GenevieveBuckley]

I think it's sensible to run the github actions CI tests on Windows, Mac and Linux, instead of just Linux.

That way we can catch any silly errors like accidentally hard coding forward slashes in cache directory paths, etc. that might work on Linux but fail on Windows (one example of a mistake I'm hoping not to make, but it would be nice to have the CI run a backup check for any silly problems like this)

[codecov]

Codecov Report

Attention: 16 lines in your changes are missing coverage. Please review.

Comparison is base (71181d4) 39.72% compared to head (5c987f6) 39.91%.
Report is 21 commits behind head on dev.

Files	Patch %	Lines
micro_sam/sample_data.py	6.66%	14 Missing ⚠️
micro_sam/sam_annotator/_widgets.py	60.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #267      +/-   ##
==========================================
+ Coverage   39.72%   39.91%   +0.19%     
==========================================
  Files          33       33              
  Lines        4179     4191      +12     
==========================================
+ Hits         1660     1673      +13     
+ Misses       2519     2518       -1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[constantinpape]

I agree that this is a good idea!
I had a look at the current test failures and they seem to be due to:

• Windows: the GUI test doesn't work. I suggest we disable it for windows.
• Mac: the CI uses MPS but the memory is so small that some tests run out of memory. I suggest we set the device to CPU for those tests. (Maybe we want to introduce an env variable so that we can tell if we are in the CI and only then set the device to CPU, so that the tests can be executed with any device in principle.)

[constantinpape]

I went ahead and finished this up @GenevieveBuckley