codecov: improve coverage upload

[ivanvc]

Align running the code coverage upload with other workflows by executing it from a Makefile target.

Update the current GitHub workflow to call this new target.

I identified this improvement while migrating this workflow to a Prow job.

Please read https://github.com/etcd-io/etcd/blob/main/CONTRIBUTING.md#contribution-flow.

[k8s-ci-robot]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.84%. Comparing base (9de211d) to head (6c86654).

Additional details and impacted files

see 19 files with indirect coverage changes

@@            Coverage Diff             @@
##             main   #19364      +/-   ##
==========================================
- Coverage   68.98%   68.84%   -0.14%     
==========================================
  Files         420      420              
  Lines       35739    35739              
==========================================
- Hits        24656    24606      -50     
- Misses       9660     9708      +48     
- Partials     1423     1425       +2     

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