gitlab-ci: drop job prefixes

gitlab-ci: drop job prefixes

It is already obvious whether the job is building a container or running
a mesa build, so let's drop that prefix so that we can see more
information on the screen (eg. in the jobs list on a pipeline page).

Signed-off-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>