ci: Use `.kokkuri:remote-context`
The latest version of Kokkuri (v2.2.0) now defines an extendable
.kokkuri:remote-context
job which sets up the build context variable
in the same way that was done here ad hoc.
Remove BUILD_FRONTEND
from .gitlab-ci.yml
. The new version of
Kokkuri can now resolve the frontend from remote config contexts hosted
on GitLab.