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 96.96%. Comparing base (40be7ee) to head (57fd3b9).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #44   +/-   ##
=======================================
  Coverage   96.96%   96.96%           
=======================================
  Files           2        2           
  Lines          99       99           
=======================================
  Hits           96       96           
  Misses          2        2           
  Partials        1        1           

Flag	Coverage Δ
macos-latest	?
oldstable	?
oldstable-macos-latest	96.96% <ø> (?)
oldstable-ubuntu-latest	96.96% <ø> (?)
oldstable-windows-latest	96.96% <ø> (?)
stable	?
stable-macos-latest	96.96% <ø> (?)
stable-ubuntu-latest	96.96% <ø> (?)
stable-windows-latest	96.96% <ø> (?)
ubuntu-latest	?

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