ci: use default_distro for coverity job
Summary
Makes it so that the default distro is always used, which avoids the problem described below.
Purpose
Previously, the coverity scheduled job seemed to fail randomly because the image that it expected to use was not available, due to not depending on the "prep" job.
Checklist
Please read https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/blob/main/CONTRIBUTING.md before opening the merge request. In particular, check that:
-
the subject for all commits is concise and explicative -
the message for all commits explains the reason for the change -
the source is properly formatted -
any relevant documentation is up to date -
you have added unit tests if applicable -
the NEWS file is updated when the change deserves to be mentioned, for example for new features, behavior changes, API deprecations, etc.