Fixes for GPU tests and GPU workflow

[BrunoLiegiBastonLiegi]

Playing around with numba threading in combination with qiboteam/qibo#1561

@andrea-pasquale this PR houses eventual fixes needed by qiboteam/qibo#1561.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.91%. Comparing base (b782805) to head (dd5bfc0).
Report is 20 commits behind head on cupy_matrices.

Additional details and impacted files

@@              Coverage Diff               @@
##           cupy_matrices     #200   +/-   ##
==============================================
  Coverage          99.91%   99.91%           
==============================================
  Files                 12       12           
  Lines               1115     1116    +1     
==============================================
+ Hits                1114     1115    +1     
  Misses                 1        1           

Flag	Coverage Δ
unittests	99.91% <100.00%> (+<0.01%)	⬆️

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[andrea-pasquale]

@BrunoLiegiBastonLiegi is there something else that needs to be done to use the selfhosted workflow in this PR after dd5bfc0?
Currently it seems to be stuck https://github.com/qiboteam/qibojit/actions/runs/13069296398/job/36467252400?pr=200

[BrunoLiegiBastonLiegi]

Not 100% sure, but probably there are actions that need to be taken at the repo level, for instance I am not sure whether this secret:

qibojit/.github/workflows/selfhosted.yml

Line 23 in dd5bfc0

repo_token: ${{ secrets.GITHUB_TOKEN }}

is available in this repo as well (@scarrazza )

[scarrazza]

I think we can remove this secret.

[BrunoLiegiBastonLiegi]

@andrea-pasquale I think we can merge this one in the meantime, such that I can update the pyproject of the qibo PR. We will address the secret problem and any other related issue directly in #199 if you agree.