Enable gcov support for codecov/codecov-action

[hugovk]

I noticed we're missing code coverage for the C files on Codecov.

For example, no C files listed at:

https://app.codecov.io/gh/python-pillow/Pillow/tree/main/src

Looks like this happened when switched bumped some workflows from v1 of the Codecov Action to v3, and others from the Codecov Bash Uploader to the v3 Action in #6281, and coverage increased, because there's better average coverage on the Python files (https://app.codecov.io/gh/python-pillow/Pillow/commit/53b6e5f4bf9a6a68e29109c457b12d17761d0035):

Now I remember I had earlier opened codecov/codecov-action#566, where the v1->v2 upgrade lost C coverage. (Codecov never replied.)

Anyway, by now, the Bash Uploader has been retired (https://github.com/codecov/codecov-bash) and there's a new Uploader (https://github.com/codecov/uploader).

Better yet, the Codecov Action has added support for gcov (used for C coverage) in v3 (codecov/codecov-action#688).

I tried enabling it for all workflows, but for some reason it only worked for the Docker one, so let's enable it there at least to get us back on track. The others will need more investigation.

[nulano]

I've created hugovk#94 to also upload C coverage for Ubuntu/macOS by removing the file: ./coverage.xml filter.

I have found that the MSYS2 does generate gcov files but the upload action is not detecting them even when removing the filter.

Cygwin does not seem to be uploading coverage at all: https://github.com/python-pillow/Pillow/actions/runs/4153272820/jobs/7184796519#step:15:26

[2023-02-11T22:28:10.252Z] ['error'] There was an error running the uploader: Error fetching git root. Please try using the -R flag. Error: Error running external program: Error: spawnSync git ENOENT

[hugovk]

Good stuff, thank you!

[hugovk]

And please could you report the MSYS2 error to https://github.com/codecov/codecov-action? Looks like the Action needs to call their uploader client differently in this case.