ci: fix codecov

[superlopuh]

As recommended here:
codecov/feedback#388

This PR makes it such that checkout fetches the two prior commits, to show the diff.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.64%. Comparing base (a173190) to head (13cc11b).
Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2650      +/-   ##
==========================================
+ Coverage   89.61%   89.64%   +0.02%     
==========================================
  Files         360      361       +1     
  Lines       46209    46252      +43     
  Branches     6986     6992       +6     
==========================================
+ Hits        41410    41462      +52     
+ Misses       3724     3719       -5     
+ Partials     1075     1071       -4     

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[math-fehr]

@superlopuh, is this working (i.e, can I merge it?)

[superlopuh]

No, please see discussion on linked issue in codecov, this PR is useful for me to keep debugging the problem. It seems like merging it would still be a good move, though, I'll hit the button and maybe open more PRs to get to the bottom of the issue.