CI: Cache test data

[effigies]

OSF seems to have been down for a bit. This should save us some time anyway.

[codecov]

Codecov Report

Merging #3459 (8dcd04b) into master (570d464) will increase coverage by 1.62%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3459      +/-   ##
==========================================
+ Coverage   63.61%   65.24%   +1.62%     
==========================================
  Files         309      309              
  Lines       40838    40838              
  Branches     5377     5377              
==========================================
+ Hits        25981    26645     +664     
+ Misses      13831    13118     -713     
- Partials     1026     1075      +49     

Flag	Coverage Δ
unittests	65.02% <ø> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
nipype/interfaces/afni/preprocess.py	81.55% <0.00%> (+0.54%)	⬆️
nipype/interfaces/spm/preprocess.py	50.28% <0.00%> (+0.84%)	⬆️
nipype/interfaces/freesurfer/model.py	63.73% <0.00%> (+1.09%)	⬆️
nipype/pipeline/engine/utils.py	74.80% <0.00%> (+1.14%)	⬆️
nipype/utils/subprocess.py	86.66% <0.00%> (+1.48%)	⬆️
nipype/interfaces/freesurfer/preprocess.py	66.04% <0.00%> (+1.65%)	⬆️
nipype/interfaces/base/core.py	87.96% <0.00%> (+2.33%)	⬆️
nipype/interfaces/base/specs.py	88.77% <0.00%> (+2.55%)	⬆️
nipype/interfaces/freesurfer/utils.py	63.25% <0.00%> (+2.77%)	⬆️
nipype/info.py	87.30% <0.00%> (+3.17%)	⬆️
... and 20 more

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