CI: add tags to all jobs

Tag all jobs with the DEFAULT_TAG variable so each instance can control
what tags the jobs have, whilst still explicitly tagging the jobs which
need specific tags (such as x86_64 for jobs which need to run x86-only
binaries)

Signed-off-by: Ross Burton <ross.burton@arm.com>

.gitlab-ci.yml

@@ -12,6 +12,8 @@ stages:
 
 # Common job fragment to get a worker ready
 .setup:
+  tags:
+    - $DEFAULT_TAG
   stage: build
   interruptible: true
   variables:
@@ -262,6 +264,8 @@ machine-coverage:
   script:
     - ./ci/check-machine-coverage
   coverage: '/Coverage: \d+/'
+  tags:
+    - x86_64
 
 metrics:
   extends: .setup