Docker sets $HOME to / when running with a UID that doesn't exist within
the container.  This not  uncommon on CI servers.

Fixes #26280

Change-Id: Ic7ff62b41403fe6e7c0cef12814667ef73f6c954
Reviewed-on: https://go-review.googlesource.com/122487
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>