net: skip TestDialCancel on linux-arm64-buildlet

These builders (on Linaro) have a different network configuration
which is incompatible with this test. Or so it seems.

Updates #15191

Change-Id: Ibfeacddc98dac1da316e704b5c8491617a13e3bf
Reviewed-on: https://go-review.googlesource.com/21901Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/net/dial_test.go

@@ -823,6 +823,9 @@ func TestDialCancel(t *testing.T) {
 	if testing.Short() && !onGoBuildFarm {
 		t.Skip("skipping in short mode")
 	}
+	if testenv.Builder() == "linux-arm64-buildlet" {
+		t.Skip("skipping on linux-arm64-buildlet; incompatible network config? issue 15191")
+	}
 
 	blackholeIPPort := JoinHostPort(slowDst4, "1234")
 	if !supportsIPv4 {