ci: cashing transformers

[Borda]

What does this PR do?

The original data folder was set as useless after periodic cleaning after the script finish, but even docstest takes long because of permanent downloading models...

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[codecov]

Codecov Report

Merging #1600 (42cbeab) into master (76d14dc) will decrease coverage by 14%.
The diff coverage is 100%.

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #1600     +/-   ##
========================================
- Coverage      86%     71%    -14%     
========================================
  Files         291     291             
  Lines       12874   12874             
========================================
- Hits        11017    9154   -1863     
- Misses       1857    3720   +1863