Tests should avoid writing to GOROOT when possible. Such writes
would fail if GOROOT is non-writeable, and it can interfere with
other tests that don't expect GOROOT to change during test execution.
Updates #28387.
Change-Id: I7d72614f218df3375540f5c2f9c9f8c11034f602
Reviewed-on: https://go-review.googlesource.com/c/go/+/284293
Trust: Dmitri Shuralyov <dmitshur@golang.org> Reviewed-by: Cherry Zhang <cherryyz@google.com> Reviewed-by: Bryan C. Mills <bcmills@google.com>