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 76.92%. Comparing base (cd9f7da) to head (71d282a).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #79   +/-   ##
=======================================
  Coverage   76.92%   76.92%           
=======================================
  Files           4        4           
  Lines         260      260           
=======================================
  Hits          200      200           
  Misses         45       45           
  Partials       15       15           

Flag	Coverage Δ
macos-latest	?
oldstable	?
oldstable-macos-latest	76.92% <ø> (?)
oldstable-ubuntu-latest	76.92% <ø> (?)
oldstable-windows-latest	76.92% <ø> (?)
stable	?
stable-macos-latest	76.92% <ø> (?)
stable-ubuntu-latest	76.92% <ø> (?)
stable-windows-latest	76.92% <ø> (?)
ubuntu-latest	?

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