tasks: Explicitly install python3-pytest-cov

[martinpitt]

Our tests need it, but it doesn't get installed as transient recommends any more.

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The 12-07 tasks container refresh lost this package, so that at least bots tests now fail:

❱❱ pytest -vv
ERROR: usage: pytest [options] [file_or_dir] [file_or_dir] [...]
pytest: error: unrecognized arguments: --cov-config=pyproject.toml
  inifile: /var/home/martin/upstream/bots/pyproject.toml
  rootdir: /var/home/martin/upstream/bots

Which can also be seen in this failure of cockpit-project/bots#7196 which ran against 2024-12-07.

I triggered a build against this branch to unblock both PRs.

[martinpitt]

The test_mock_pr failure here is fixed by cockpit-project/bots#7196