CI: Fall back to older codecov version

[stotko]

Version 5.0.4 which has been released yesterday updated its internal wrapper code which, however, introduced a regression and prevents reports from being displayed. The reason seems that the git commits get messed up but we cannot do much on our side to fix this. Fall back to a slightly older version to workaround this issue until it has been fixed upstream.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.93%. Comparing base (4907180) to head (87d29f8).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #451   +/-   ##
=======================================
  Coverage   96.93%   96.93%           
=======================================
  Files          33       33           
  Lines        2672     2672           
=======================================
  Hits         2590     2590           
  Misses         82       82           

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