cmd/go: use objdir as consistent variable name for per-package work dir

Before it was obj, but if you don't have everything paged in
that sounds a bit like an object file. Use objdir, which is more
clearly a directory and also matches the Action.Objdir struct field.

Change-Id: I268042800f9ca05721814d7f18c728acb4831232
Reviewed-on: https://go-review.googlesource.com/56277Reviewed-by: Ian Lance Taylor <iant@golang.org>