tests: disable gpg unknown_key() test on Windows as well

Follows up 7552f939c6 "tests: disable most gpg integration tests on Windows."
I couldn't find this test failing in a few samples before, but it does now.

lib/tests/test_gpg.rs

@@ -156,6 +156,7 @@ fn gpg_signing_roundtrip_explicit_key() {
 }
 
 #[test]
+#[cfg_attr(windows, ignore = "stuck randomly on Windows CI #3140")] // FIXME
 fn unknown_key() {
     gpg_guard!();