disable pip cache due to slow upload and download

[xyb]

Github Actions' caching infrastructure is too slow to use recently, so temporary disable it to improve the speed of CI:

• pip install: 37s
• pip cache upload: 11m 37s
• pip cache download: 3m 53s

[codecov-commenter]

Codecov Report

Merging #28 (4c9d1a8) into main (f72539f) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main      #28   +/-   ##
=======================================
  Coverage   90.28%   90.28%           
=======================================
  Files           6        6           
  Lines         700      700           
=======================================
  Hits          632      632           
  Misses         68       68           

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