Bump deb build to test stage in gitlab ci config

This puts it at the stame stage as the flatpak package and skips one
build that will always fail if the Ubuntu one fails for legit reasons.

.gitlab-ci.yml

@@ -59,7 +59,7 @@ ubuntu:
     - $INSTALL_CMD
 
 deb-package:
-  stage: build
+  stage: test
   image: ubuntu:rolling
   before_script:
     - apt-get update