CI: use DEFAULT_TAG as the default ACS_TAG

This stops the job being stuck if the runners will only take jobs that
have been tagged.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>

.gitlab-ci.yml

@@ -10,7 +10,7 @@ variables:
   # The default machine tag for the build jobs
   DEFAULT_TAG: ""
   # The machine tag for the ACS test jobs
-  ACS_TAG: ""
+  ACS_TAG: "$DEFAULT_TAG"
   # The directory to use as the persistent cache (the root for DL_DIR, SSTATE_DIR, etc)
   CACHE_DIR: $CI_BUILDS_DIR/persist
   # The container mirror to use