cmd/go: skip TestGoGenerateEnv on Plan 9

TestGoGenerateEnv was added in CL 16537.
However, Plan 9 doesn't have the env command.

Change-Id: I5f0c937a1b9b456dcea41ceac7865112f2f65c45
Reviewed-on: https://go-review.googlesource.com/16690Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: David du Colombier <0intro@gmail.com>

src/cmd/go/go_test.go

@@ -2036,8 +2036,9 @@ func TestGoGenerateRunFlag(t *testing.T) {
 }
 
 func TestGoGenerateEnv(t *testing.T) {
-	if runtime.GOOS == "windows" {
-		t.Skip("skipping because windows does not have the env command")
+	switch runtime.GOOS {
+	case "plan9", "windows":
+		t.Skipf("skipping because %s does not have the env command", runtime.GOOS)
 	}
 	tg := testgo(t)
 	defer tg.cleanup()