net: don't run TestLookupDotsWithLocalSource in short mode

Do run it on the builders, though.

Fixes #15881.

Change-Id: Ie42204d553cb18547ffd6441afc261717bbd9205
Reviewed-on: https://go-review.googlesource.com/24111
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/net/lookup_test.go

@@ -453,6 +453,10 @@ func TestLookupDotsWithLocalSource(t *testing.T) {
 		t.Skip("IPv4 is required")
 	}
 
+	if testenv.Builder() == "" {
+		testenv.MustHaveExternalNetwork(t)
+	}
+
 	for i, fn := range []func() func(){forceGoDNS, forceCgoDNS} {
 		fixup := fn()
 		if fixup == nil {