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 66.34%. Comparing base (6613744) to head (77308df).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #210      +/-   ##
==========================================
+ Coverage   65.46%   66.34%   +0.88%     
==========================================
  Files          29       30       +1     
  Lines        2849     2927      +78     
==========================================
+ Hits         1865     1942      +77     
- Misses        821      822       +1     
  Partials      163      163              

Flag	Coverage Δ
macos-latest	?
oldstable	?
oldstable-macos-latest	65.46% <ø> (?)
oldstable-ubuntu-latest	65.46% <ø> (?)
oldstable-windows-latest	66.34% <ø> (?)
stable	?
stable-macos-latest	65.46% <ø> (?)
stable-ubuntu-latest	65.46% <ø> (?)
stable-windows-latest	66.34% <ø> (?)
ubuntu-latest	?

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.