Codecov integration needs fixing

[simondeziel]

The codecov integration seems broken (https://github.com/canonical/pylxd/actions/runs/11149658300/job/30989023266#step:6:125):

...
==> Detecting CI provider
    GitHub Actions CI Detected
==> Preparing upload
==> Processing gcov (disable by -X gcov)
==> Collecting reports
    Generating coverage xml reports for Python
    + /home/runner/work/pylxd/pylxd/coverage.xml bytes=121416
==> Uploading
    .url https://codecov.io/
    .query commit=9d485a4d565528f7eecf40819731e451c4e9f5d5&branch=main&build_url=http%3A%2F%2Fgithub.com%2Fcanonical%2Fpylxd%2Factions%2Fruns%2F11149658300&service=github-actions&build=11149658300&slug=canonical%2Fpylxd&package=py2.1.13
    Gzipping contents..
    Compressed contents to 7761 bytes
    Pinging Codecov...
    Retrying 1/5 in 2s..
    Retrying 2/5 in 2s..
    Retrying 3/5 in 2s..
    Retrying 4/5 in 2s..
    Retrying 5/5 in 2s..
    Direct to s3 failed. Using backup v2 endpoint.
    Uploading to Codecov...
    Retrying 1/5 in 2s..
    Retrying 2/5 in 2s..
    Retrying 3/5 in 2s..
    Retrying 4/5 in 2s..
    Retrying 5/5 in 2s..
    {'detail': ErrorDetail(string='Rate limit reached. Please upload with the Codecov repository upload token to resolve issue. Expected time to availability: 518s.', code='throttled')}
Error: 429 Client Error: Too Many Requests for url: https://codecov.io/upload/v2?commit=9d485a4d565528f7eecf40819731e451c4e9f5d5&branch=main&build_url=http%3A%2F%2Fgithub.com%2Fcanonical%2Fpylxd%2Factions%2Fruns%2F11149658300&service=github-actions&build=11149658300&slug=canonical%2Fpylxd&package=py2.1.13
Tip: See all example repositories: https://github.com/codecov?query=example

This upload is done in the matrix testing which feels wrong. It should probably only be done for 1 job.

It might be a good time to switch to using https://github.com/codecov/codecov-action and we might get away without needing any upload token as it's OSS, see https://docs.codecov.com/docs/codecov-uploader#supporting-token-less-uploads-for-forks-of-open-source-repos-using-codecov for details.

Update: it seems a token will been needed and will require to be stored as a GH secret.