ci: Set the correct FDO_UPSTREAM_REPO for ci-fairy

We reuse the container images from the gstreamer-rs repo so we are
setting the FDO_UPSTREAM_REPO based on that for the images.

However ci-fairy also relies on that variable for a couple of things
so we need to tell it where to find the real repository to use.

.gitlab-ci.yml

@@ -319,6 +319,8 @@ check commits:
   extends: '.debian:12-stable'
   stage: "lint"
   needs: []
+  variables:
+    FDO_UPSTREAM_REPO: "gstreamer/gst-plugins-rs"
   script:
     - ci-fairy check-commits --textwidth 0 --no-signed-off-by
     - ci/check-for-symlinks.sh