dashboard/env: update linux builder Dockerfile to use git, update builder

Uses a builder that can build both hg & git. (Should fix the git
subrepos on the main dashboard currently)

Change-Id: I3af96e35d40f6397625730b812a288f36622cb8a
Reviewed-on: https://go-review.googlesource.com/1203
Reviewed-by: Andrew Gerrand <adg@golang.org>