We should avoid writing temp files to GOROOT, since it might be readonly.

Fixes #23881

Change-Id: Iaa38ec404b303f0cf27fdfb7daf1ddd60fd5d1c9
GitHub-Last-Rev: de0211df8474cc3bbef40f792e2f85b3b6ee259c
GitHub-Pull-Request: golang/go#24238
Reviewed-on: https://go-review.googlesource.com/98517
Run-TryBot: Giovanni Bajo <rasky@develer.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>