TST: Set default branch for codecov

[pllim]

This PR is to set default branch for codecov to prevent silent config regression upstream.

Tasks

• If you have a specific reviewer in mind, tag them.
• add a build milestone, i.e. Build 12.0 (use the latest build if not sure)
• Does this PR change user-facing code / API? (if not, label with no-changelog-entry-needed)
  • write news fragment(s) in changes/: echo "changed something" > changes/<PR#>.<changetype>.rst (see changelog readme for instructions)
  • update or add relevant tests
  • update relevant docstrings and / or docs/ page
  • start a regression test and include a link to the running job (click here for instructions)
    • Do truth files need to be updated ("okified")?
      • after the reviewer has approved these changes, run okify_regtests to update the truth files
• if a JIRA ticket exists, make sure it is resolved properly

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 0.00%. Comparing base (a1763ec) to head (01bac21).
⚠️ Report is 422 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #9648   +/-   ##
=====================================
  Coverage   0.00%   0.00%           
=====================================
  Files        367     367           
  Lines      37313   37313           
=====================================
  Misses     37313   37313           

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[pllim]

Not sure if the 0% will get fixed until after merge.

[pllim]

Also does this mean your fix on codecov admin panel didn't work, @jhunkeler ?