ci: Fix build-previous-qemu when the version tag is absent

ci: Fix build-previous-qemu when the version tag is absent

Stefan reports that during QEMU release, pushing a series with the
VERSION bump commit, but not pushing the new git tag in the same
command will cause a failure of the build-previous-qemu job at the git
fetch step.

Since the job is intended to produce a build of the previous QEMU
version for consumption by the migration-compat-* jobs, there's no
reason to produce a build of the release commit because the migration
job would end up testing the release against itself.

Skip the job when VERSION contains the newly release version number.

I'm opting for 'exit 0' for both the build and the test jobs because
allow_failure would mask any real error in the jobs. It also avoids
having an orange ! on every release pipeline.

Reported-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>
Link: https://lore.kernel.org/r/20250513133353.23022-3-farosas@suse.de
Signed-off-by: Peter Xu <peterx@redhat.com>