Restore covr

[MichaelChirico]

Closes #6479.

First tried reverting the old GHA, but I think there was a cacheing issue there as we continued to segfault even with old R. Therefore I cribbed the code-coverage GHA from dplyr which is structurally a bit different. With that change our codecov passes again, once I pinned the R version to <4.4.

I also generated+added a CODECOV_TOKEN under our repo secrets.

[MichaelChirico]

OK, we have covr() working 🎉 there's still some stuff to iron out about tokens & communicating to codecov, hopefully that part's easier:

Error: Error: Codecov token not found. Please provide Codecov token with -t flag.
Warning: Codecov: Failed to properly create commit: The process '/home/runner/work/_actions/codecov/codecov-action/v4/dist/codecov' failed with exit code 1

[MichaelChirico]

(side note: added @tdhock @jangorecki and @ben-schwen as admins in the codecov.io "project" for Rdatatable)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.02%. Comparing base (6f3fc8d) to head (ad18c93).
Report is 402 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6540      +/-   ##
==========================================
- Coverage   97.51%   97.02%   -0.49%     
==========================================
  Files          80       81       +1     
  Lines       14979    14708     -271     
==========================================
- Hits        14607    14271     -336     
- Misses        372      437      +65     

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[MichaelChirico]

Looks like we've lost 0.44% coverage since the last upload 6f3fc8d :)

[tdhock]

not bad thanks