cmd/link: remove GOROOT write in TestBuildForTvOS

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>