Disable caching on venv tests

[tovrstra]

The caching in Github Actions seems to break easily. (This is probably due to limited storage for the caches, not sure.) I'm keeping cache in place for the conda test on the master branch, but not for venv tests which run for each PR.

I'll merge this as soon as tests pass because this issue affects all other PRs.

[codecov]

Codecov Report

Merging #264 (4a868cd) into master (333126a) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #264   +/-   ##
=======================================
  Coverage   95.64%   95.64%           
=======================================
  Files          74       74           
  Lines        8214     8249   +35     
  Branches     1068     1072    +4     
=======================================
+ Hits         7856     7890   +34     
  Misses        164      164           
- Partials      194      195    +1     

Impacted Files	Coverage Δ
iodata/formats/sdf.py	98.50% <0.00%> (-1.50%)	⬇️
iodata/periodic.py	100.00% <0.00%> (ø)
iodata/test/test_sdf.py	100.00% <0.00%> (ø)

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