gha: add codecov

[thaJeztah]

No description provided.

[thaJeztah]

OK, I think this is working;

=> Running upload-coverage
      ./codecov  upload-coverage --git-service github --sha f5305cea0432e651a4d785db82cb9541cc900d88 --branch thaJeztah:add_codecov --gcov-executable gcov
info - 2024-12-18 17:38:05,181 -- ci service found: github-actions
warning - 2024-12-18 17:38:05,190 -- xcrun is not installed or can't be found.
warning - 2024-12-18 17:38:05,192 -- No gcov data found.
warning - 2024-12-18 17:38:05,192 -- coverage.py is not installed or can't be found.
info - 2024-12-18 17:38:05,201 -- Found 7 coverage files to report
info - 2024-12-18 17:38:05,201 -- > /home/runner/work/sys/sys/reexec/coverage.txt
info - 2024-12-18 17:38:05,202 -- > /home/runner/work/sys/sys/mount/coverage.txt
info - 2024-12-18 17:38:05,202 -- > /home/runner/work/sys/sys/mountinfo/coverage.txt
info - 2024-12-18 17:38:05,202 -- > /home/runner/work/sys/sys/sequential/coverage.txt
info - 2024-12-18 17:38:05,202 -- > /home/runner/work/sys/sys/user/coverage.txt
info - 2024-12-18 17:38:05,202 -- > /home/runner/work/sys/sys/signal/coverage.txt
info - 2024-12-18 17:38:05,202 -- > /home/runner/work/sys/sys/symlink/coverage.txt
info - 2024-12-18 17:38:05,876 -- Your upload is now processing. When finished, results will be available at: https://app.codecov.io/github/moby/sys/commit/f5305cea0432e651a4d785db82cb9541cc900d88
info - 2024-12-18 17:38:06,036 -- Process Upload complete

[thaJeztah]

These were copied from what we had in moby/moby and docker/cli. Not sure if they're all needed, but at least basic functionality works. We could enable it leaving comments, but they are sometimes a bit noisy, so I left that off for now