Respect read-only cache jobs when specified in .gitlab-ci.yml
Per https://gitlab.com/gitlab-org/gitlab-ce/issues/33772#note_33008526 , we're going to introduce key to the cache section of jobs that specifies cache files should be downloaded, but not uploaded. We provisionally call this readonly: true.
This issue is to track implementation in gitlab-ci-multi-runner
It shouldn't be too difficult - we have seperate cache download and upload steps in shells/abstract.go, we just need to make the latter conditional on this key.