Store relative file paths in the data file

By default, coverage data contains absolute paths for the test suite. During CI,
coverage data from various runners is combined in an environment where these
paths do not exist. This results in errors like the following:

  No source for code:
  '/Users/runner/work/cookiecutter-hypermodern-python-instance/cookiecutter-hypermodern-python-instance/tests/__init__.py'.
  Aborting report output, consider using -i.

The `coverage.paths` setting does not help with the test suite: Unlike the code
under `src`, the test suite is not installed to `site-packages`, so we'd need to
either hardcode paths from individual runners, or use overly generic wildcards.

Instead, `coverage.run.relative_files` can be used to store the paths relative
to the repository. We still need `coverage.paths` for `src` though, because
these file paths still include the path into the various Nox environments.

pyproject.toml

@@ -52,6 +52,7 @@ source = ["src", "*/site-packages"]
 
 [tool.coverage.run]
 branch = true
+relative_files = true
 source = ["cookiecutter_hypermodern_python_instance", "tests"]
 
 [tool.coverage.report]