setup-test-env and artifacts

Someone sometime ago added setup-test-env that does assets:precompile.

This is great, because it reduces a little the compute time, but also this is terrible, because:

1. we transfer artifacts to GitLab,
2. for each job we pull these artifacts from GitLab,

It seems that artifacts for setup-test-env weight about 20MB. This gives:

• ~900MB extra egress transfer per pipeline on branch,
• ~1400MB extra egress transfer per pipeline on master.

So, we are reducing (a little) the compute time for the cost of egress traffic.

It would help to have a local artifacts, that newer hit gitlab, and are transfered between runners internally. However, we are far from having that.

cc @stanhu @rymai @pcarranza