ci(goreleaser): only generate signature for checksums.txt

[phm07]

If we only want to generate a .sig release artifact for checksums.txt, we need to set artifact: checksum.
See https://goreleaser.com/customization/sign/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.64%. Comparing base (26e799b) to head (549fe3c).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1366      +/-   ##
==========================================
+ Coverage   72.60%   72.64%   +0.03%     
==========================================
  Files         300      300              
  Lines       11032    11051      +19     
==========================================
+ Hits         8010     8028      +18     
+ Misses       2130     2129       -1     
- Partials      892      894       +2     

Flag	Coverage Δ
e2e	51.33% <ø> (-0.09%)	⬇️
unit	67.58% <ø> (+0.04%)	⬆️

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