ci: disable anholt's farm

Its http cache proxy has been returning `curl: (52) Empty reply from
server` for a while and rebooting it didn't help, so turn it off for now.

Suggested-by: Emma Anholt <emma@anholt.net>
Signed-off-by: Eric Engestrom <eric@igalia.com>

.gitlab-ci.yml

@@ -35,7 +35,7 @@ variables:
   MICROSOFT_FARM: "online"
   LIMA_FARM: "online"
   IGALIA_FARM: "online"
-  ANHOLT_FARM: "online"
+  ANHOLT_FARM: "offline"
   VALVE_FARM: "online"
   AUSTRIANCODER_FARM: "online"  # only etnaviv GPUs