internal/lsp/regtest: Skip failing/flaky TestRegenerateCgo

Something is making this test deterministically fail in some
environments, such as @bcmills' desktop.

Skip it while I build go at tip and debug.

Updates golang/go#39135

Change-Id: I1bf8c55c5cfca471a904de85936f504313094807
Reviewed-on: https://go-review.googlesource.com/c/tools/+/234480
Run-TryBot: Robert Findley <rfindley@google.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

internal/lsp/regtest/cgo_test.go

@@ -11,6 +11,7 @@ import (
 )
 
 func TestRegenerateCgo(t *testing.T) {
+	t.Skip("This test fails in some environments: see golang.org/issues/39135")
 	testenv.NeedsTool(t, "cgo")
 
 	const workspace = `