Specify branch for GitHub Actions badge and fix link

[pitdicker]

I think it makes more sense for the GitHub Actions badge to match the latest run on the branch that that particular readme is on.
For the main branch I'll make it point to the main with the next merge.

This also fixes the link, that seems to have been broken for quite a while.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (94f9738) 91.72% compared to head (850c55e) 91.72%.

❗ Current head 850c55e differs from pull request most recent head ab57b81. Consider uploading reports for the commit ab57b81 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1388   +/-   ##
=======================================
  Coverage   91.72%   91.72%           
=======================================
  Files          38       38           
  Lines       17267    17267           
=======================================
  Hits        15839    15839           
  Misses       1428     1428           

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[pitdicker]

@djc Just saw your comment on switching the main branch. If we go that route we may only want to fix the link here.

[djc]

See #1390, so this might need some changes?