tests: disable most gpg integration tests on Windows

These tests often stuck on Windows CI for unknown reasons. Let's mark them
ignored for the moment. The unknown_key test is allowed because it somehow
appears to pass.

https://github.com/martinvonz/jj/actions/runs/8009950119/job/21879789008?pr=3123#step:7:1487

#3140

lib/tests/test_gpg.rs

@@ -95,6 +95,7 @@ fn backend(env: &GpgEnvironment) -> GpgBackend {
 }
 
 #[test]
+#[cfg_attr(windows, ignore = "stuck randomly on Windows CI")] // FIXME
 fn gpg_signing_roundtrip() {
     gpg_guard!();
 
@@ -121,6 +122,7 @@ fn gpg_signing_roundtrip() {
 }
 
 #[test]
+#[cfg_attr(windows, ignore = "stuck randomly on Windows CI")] // FIXME
 fn gpg_signing_roundtrip_explicit_key() {
     gpg_guard!();
 
@@ -187,6 +189,7 @@ fn unknown_key() {
 }
 
 #[test]
+#[cfg_attr(windows, ignore = "stuck randomly on Windows CI")] // FIXME
 fn invalid_signature() {
     gpg_guard!();