Fix GitHub actions MacOS

[brosaplanella]

Description

Looks like our issue with the MacOs tests evolved and now it doesn't randomly fail anymore. I have found that adding solves it
brew unlink gcc@8 gcc@9 (I hope it doesn't break anything down the line...).

[codecov]

Codecov Report

Merging #1319 (b2cd13a) into develop (cd833d0) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #1319   +/-   ##
========================================
  Coverage    98.11%   98.11%           
========================================
  Files          272      272           
  Lines        15340    15340           
========================================
  Hits         15051    15051           
  Misses         289      289           

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[valentinsulzer]

Looks good, thanks @brosaplanella

[tlestang]

Thanks Ferran! Would it be possible to add one or two comments explaining why these lines where added? With a link with relevant references if this is inspired from something else.

[tlestang]

Two relevant issues are:

• brew upgrade fails for Python's 2to3 conflict
• brew upgrade fails on gcc

[brosaplanella]

Two relevant issues are:

• brew upgrade fails for Python's 2to3 conflict
• brew upgrade fails on gcc

I have now added the extra information. I only implemented the fix for gcc, which was basically what it was suggested by the error message. Let me know if I should add anything else.