dashboard/builder: change default gcpath back to hg for now

We're doing this so we can update the main builders to fix the
problems with subrepos. The coordinator doesn't know how to
pass -gcpath to this.

We can change it back to git after Go 1.4 is out.

Change-Id: I3924a34ac974cb77954acea4772ef0f52e36fa45
Reviewed-on: https://go-review.googlesource.com/1202
Reviewed-by: Andrew Gerrand <adg@golang.org>