Try fixing the ffi_yajl not found issue by using a new cache-key
Fixes https://gitlab.com/gitlab-org/omnibus-gitlab/issues/2186
Not exactly sure how. The old runner default cache key, was job_name/branch, the new default in 9.0 was just the key 'default' Here I have changed it to just the job name, so we re-use the cache accross branches for now.
I'm not sure why some of these are able to use the cache, and some are not.