Let GitLab provide a drop-down for DISABLE_CACHE

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

.gitlab-ci.yml

@@ -33,6 +33,9 @@ variables:
       cache at this step to unblock dashboard updates.
       See https://phabricator.apertis.org/T9440
     value: "no"
+    options:
+      - "no"
+      - "yes"
   FILTER_ON_CACHE:
     description: |
       Fetching upstream sources data for all Debian packages (~ 38100 pkgs)