CI: Run Pages build on main

Predictably, there was one thing I forgot with !600, which was to fix up
the CI rules for the transition. Oops.

Signed-off-by: Daniel Stone <daniels@collabora.com>

.gitlab-ci.yml

@@ -168,4 +168,4 @@ pages:
     paths:
     - public
   only:
-    - master
+    - main