cmd/go/internal/work: skip TestRespectSetgidDir on iOS

Attempting to set the SetGID bit on iOS 11 fails with a permission
error. Skip the test.

Change-Id: Idac59750447d668091c44fe7cc5ee178014e0e1d
Reviewed-on: https://go-review.googlesource.com/66490
Run-TryBot: Elias Naur <elias.naur@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/go/internal/work/build_test.go

@@ -175,8 +175,13 @@ func pkgImportPath(pkgpath string) *load.Package {
 // directory.
 // See https://golang.org/issue/18878.
 func TestRespectSetgidDir(t *testing.T) {
-	if runtime.GOOS == "nacl" {
+	switch runtime.GOOS {
+	case "nacl":
 		t.Skip("can't set SetGID bit with chmod on nacl")
+	case "darwin":
+		if runtime.GOARCH == "arm" || runtime.GOARCH == "arm64" {
+			t.Skip("can't set SetGID bit with chmod on iOS")
+		}
 	}
 
 	var b Builder