Run coverage only on main and tags

[kp992]

Currently, it triggers coverage run on each commit push.

Ex: https://github.com/kp992/shell/actions
cc @wolfv.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 63.20%. Comparing base (05fbd3d) to head (7a47c55).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #261   +/-   ##
=======================================
  Coverage   63.20%   63.20%           
=======================================
  Files          33       33           
  Lines        3712     3712           
=======================================
  Hits         2346     2346           
  Misses       1366     1366           

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