CI: use OIDC login for CodeCov

[pjonsson]

CodeCov requires some kind of login
for protected branches, so enable
the OIDC login so the comparison
point isn't stuck 20+ commits
behind the latest main.

Also avoid failing the CI if
CodeCov fails since there is
a fair bit of downtime for that
service

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.00%. Comparing base (50e7109) to head (d804b3d).
⚠️ Report is 29 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #125      +/-   ##
==========================================
+ Coverage   92.27%   95.00%   +2.72%     
==========================================
  Files           6        6              
  Lines         686      760      +74     
==========================================
+ Hits          633      722      +89     
+ Misses         53       38      -15     

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