Uses/cabal.mk: Remove the --offline flag from the do-build command

It is a fatal error now if Cabal is passed --offline and there are VCS
dependencies in the cabal.project
This change is no-op for Poudriere builds since it shuts down networking for
the build stage by default.

(cherry picked from commit fe95e6b94dc2ce4908578fbe4d697245c750a263)