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>

src/cmd/link/link_test.go

@@ -320,6 +320,7 @@ func TestBuildForTvOS(t *testing.T) {
 	}
 
 	link := exec.Command(CC[0], CC[1:]...)
+	link.Args = append(link.Args, "-o", filepath.Join(tmpDir, "a.out")) // Avoid writing to package directory.
 	link.Args = append(link.Args, ar, filepath.Join("testdata", "testBuildFortvOS", "main.m"))
 	if out, err := link.CombinedOutput(); err != nil {
 		t.Fatalf("%v: %v:\n%s", link.Args, err, out)