cmd/go: don't run TestInstalls in short mode

It changes GOROOT, so we shouldn't run it in short mode.  Also, it's
fairly slow.

Update #11779.

Change-Id: I3d3344954cf9b2ac70070c878a67cb65ac8fd85c
Reviewed-on: https://go-review.googlesource.com/12364
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>

src/cmd/go/go_test.go

@@ -1207,6 +1207,10 @@ func TestGoGetNonPkg(t *testing.T) {
 }
 
 func TestInstalls(t *testing.T) {
+	if testing.Short() {
+		t.Skip("don't install into GOROOT in short mode")
+	}
+
 	tg := testgo(t)
 	defer tg.cleanup()
 	tg.parallel()