ci: specify/run only the gcov pluin

ci: specify/run only the gcov pluin

The default has changed with codecov v5, where all existing plugins are
attempted. In practise we only need gcov, which seems to work file.

Specify it, saving us the following warning:

  coverage.py is not installed or can't be found.

Initially I tried adding it to the Ubuntu image, only to notice they
lack the looked for `coverage.py` executable and instead have
`python{version.minor}-coverage.py`.

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Link: https://github.com/kmod-project/kmod/pull/448
Signed-off-by: Lucas De Marchi <ldemarchi@kernel.org>