ci: reduce priority of nightly pipeline jobs from 50 to 45

This means the fork jobs now have clear priority over nightly jobs,
although it might not matter much as nightly jobs are mostly
long-running jobs and pre-merge & forks will have to wait regardless of
priority.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/32093>

.gitlab-ci.yml

@@ -47,6 +47,7 @@ workflow:
     # nightly pipeline
     - if: &is-scheduled-pipeline $CI_PIPELINE_SOURCE == "schedule"
       variables:
+        JOB_PRIORITY: 45
         # (some) nightly builds perform LTO, so they take much longer than the
         # short timeout allowed in other pipelines.
         # Note: 0 = infinity = gitlab's job `timeout:` applies, which is 1h