This should give a nice name to distribution archives so that they are
not all called `artifacts.zip`. Names will better describe their
contents (target OS or source and short commit hash, because for CI
builds, it's important to know which commit is being tested).

Also replace CI_COMMIT_REF_NAME in other artifact names by
CI_COMMIT_REF_SLUG. Otherwise if a branch has a slash (quite common in
branch names), only the part after the last slash is used for archive
naming.

Finally immediately exits from dependency build with error code (!= 0)
if `crossroad install` command failed.