ci: Parametrize $GITLAB_RULES_URL on $OSNAME

Make downstream customization slightly easier by moving the `apertis`
bit to a separate and cleaner to customize variable, which could be
easily set at the group level.

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>

.gitlab-ci.yml

@@ -8,7 +8,8 @@ default:
       - script_failure
 
 variables:
-  GITLAB_RULES_URL: $CI_SERVER_URL/infrastructure/apertis-infrastructure/-/raw/main/gitlab-scripts/rulez.yaml
+  OSNAME: apertis
+  GITLAB_RULES_URL: $CI_SERVER_URL/infrastructure/$OSNAME-infrastructure/-/raw/main/gitlab-scripts/rulez.yaml
   DEBUG:
     description: Set to any non-empty value to enable debug output
     value: ""