[bugfix] Use new the 2020 pip dependency resolver in the bootstrap script

[vkarak]

This solves the following error when installing ReFrame's dependencies with the bootstrap script:

ERROR: After October 2020 you may experience errors when installing or updating packages. This is because pip will change the way that it resolves dependency conflicts.

We recommend you use --use-feature=2020-resolver to test your packages with the new resolver before it becomes the default.

This PR also disallows pip cache in the bootstrap script.

[codecov-commenter]

Codecov Report

Merging #1461 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1461   +/-   ##
=======================================
  Coverage   91.70%   91.70%           
=======================================
  Files          82       82           
  Lines       12802    12802           
=======================================
  Hits        11740    11740           
  Misses       1062     1062           

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[vkarak]

@jenkins-cscs retry daint