Use a build cache separate from the default user cache,
one that will be wiped out during startup, so that make.bash
continues to start from a clean slate.

Change-Id: I38733991015c66efb89fc170c71701b1dd9de28d
Reviewed-on: https://go-review.googlesource.com/75291
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>