GITLAB_USER_* variables represent the user *that triggered* the job,
which might not match the user that owns the namespace of the fork.

This happens frequently with rebase, where the one who triggers the
rebase is the one that appears in the GITLAB_USER_* vars.

Instead use the CI_PROJECT_NAMESPACE variable to search the user's
projects. If CI_PROJECT_NAMESPACE has the value of a Group namespace
search_user_namespace returns 404.