Fix submodules error checking in prepare-gerrit-repos

The prepare-gerrit-repos role has an error checking task that ensures
git submodules are properly checked out for projects under test. There
was a problem with this check in that it would check all projects in the
zuul projects list against the valid submodule criteria even if that
particular project is not a submodule.

Not being a submodule evaluated to not being a valid submodule. This
makes sense except for in these cases we don't care to check submodule
critera for non submodules. Simply skip this check for repos that are
not submodules.

Bug: Issue 40015618
Change-Id: Ia16bf721d4d6d65be23ab54f3c41c05faa20a759

roles/prepare-gerrit-repos/tasks/repo.yaml

diff --git a/roles/prepare-gerrit-repos/tasks/repo.yaml b/roles/prepare-gerrit-repos/tasks/repo.yaml
index 2d542dc..f3bf26f 100644
--- a/roles/prepare-gerrit-repos/tasks/repo.yaml
+++ b/roles/prepare-gerrit-repos/tasks/repo.yaml
@@ -30,6 +30,7 @@
 
 - name: Check for unsatisfiable source repo condition
   when:
+    - "project_in_gitmodules"
     - "project.canonical_name != zuul.project.canonical_name"
     - "not project_branch_exists or not tracking_branch"
     - "repo_has_dependent_change"