fix release workflow, add sign workflow

[janfb]

the last step of signing the distribution files and uploading them to the GH Release assets failed because it was trying to create the release even though it was already created manually.

• this is fixed now in the old release file
• a new workflow is added to trigger the signing and uploading for an existing release manually.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.75%. Comparing base (3cf75c0) to head (36e79aa).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1663      +/-   ##
==========================================
- Coverage   87.89%   83.75%   -4.14%     
==========================================
  Files         134      134              
  Lines       11168    11168              
==========================================
- Hits         9816     9354     -462     
- Misses       1352     1814     +462     

Flag	Coverage Δ
unittests	83.75% <ø> (-4.14%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.
see 30 files with indirect coverage changes