ci: fixed codecov upload

[fredbi]

Fixed broken test coverage upload to codecov with codecov-action@v5.

* fixed tokenless upload (now supported with codecov v5)
* fixed flags for codecov
* fixed coverage for windows (now use bash instead of powershell)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.83%. Comparing base (b57b776) to head (b15f8bc).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #344   +/-   ##
=======================================
  Coverage   81.83%   81.83%           
=======================================
  Files          49       49           
  Lines        4073     4073           
=======================================
  Hits         3333     3333           
  Misses        628      628           
  Partials      112      112           

Flag	Coverage Δ
1.20	?
1.20-macos-latest	81.83% <ø> (?)
1.20-ubuntu-latest	81.83% <ø> (?)
1.20-windows-latest	81.83% <ø> (?)
macos-latest	?
oldstable	?
oldstable-macos-latest	81.83% <ø> (?)
oldstable-ubuntu-latest	81.83% <ø> (?)
oldstable-windows-latest	81.83% <ø> (?)
stable	?
stable-macos-latest	81.83% <ø> (?)
stable-ubuntu-latest	81.83% <ø> (?)
stable-windows-latest	81.83% <ø> (?)
ubuntu-latest	?

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