Fix another miri fail in hugr-model

hugr-model/tests/text.rs

@@ -7,6 +7,7 @@ fn roundtrip(source: &str) -> String {
 }
 
 #[test]
+#[cfg_attr(miri, ignore)] // Opening files is not supported in (isolated) miri
 pub fn test_declarative_extensions() {
     insta::assert_snapshot!(roundtrip(include_str!("fixtures/model-decl-exts.edn")))
 }