Fix only_rerun marker precedence test

[janmrow]

test_only_rerun_flag_in_flaky_marker builds CLI arguments but doesn't pass them to runpytest(). This means its marker-vs-CLI cases never exercise the CLI option.

Pass the arguments to runpytest() so the test verifies that marker-level only_rerun takes precedence over the CLI value.

To confirm the gap, I temporarily reversed the precedence. The original test still passed, while the corrected test failed as expected.

[janmrow]

This is a test-only one-line fix, so I left out a changelog entry in line with the “unless the change is trivial” guidance in CONTRIBUTING.rst. I’m happy to add a changes/348.misc.rst entry if preferred.