Codecov upload from CI is broken

[FreezyLemon]

Since #3348, CI can't upload codecov stats for the master branch:

Error: Codecov token not found. Please provide Codecov token with -t flag.

example run

The codecov repo mentions this as a breaking change:

Tokenless uploading is unsupported. However, PRs made from forks to the upstream public repos will support tokenless (e.g. contributors to OS projects do not need the upstream repo's Codecov token)

This can be fixed by creating/getting an upload token on codecov, setting it as a secret (e.g. CODECOV_TOKEN) in the GitHub repo and then adding it to the CI job:

uses: codecov/codecov-action@v4
with:
    files: lcov.info
    token: ${{ secrets.CODECOV_TOKEN }} # this is new