ci: Cancel redundant jobs for new pipelines

When a new commit is made on an existing branch, to cancel the jobs that
have already started `interruptible` needs to be set to `true`. Before
`build` and `test` were combined to a single job, jobs were were waiting
for the runners and hence GitLab CI was able to cancel the queued jobs
(as it was not started yet). With CI optimized more jobs will be running
in parallel and hence to end a running pipeline faster, it will need to
cancel the running jobs.

Reference: https://docs.gitlab.com/ee/ci/yaml/#interruptible



Signed-off-by: Saheer Babu <saheer.babu@arm.com>