cmd/go/internal/work: disallow testgo binary from installing to GOROOT

Installing to GOROOT makes tests non-parallelizable, since each test
depends on the installed contents of GOROOT already being up-to-date
and may reasonably assume that those contents do not change over the
course of the test.

Fixes #37573
Updates #30316

Change-Id: I2afe95ad11347bee3bb7c2d77a657db6d691cf05
Reviewed-on: https://go-review.googlesource.com/c/go/+/225897


Reviewed-by: Michael Matloob <matloob@golang.org>