cli/test: Skip `rad_diff` on macOS

macOS's `sed` requires an argument for `-i`, which we don't provide
in the example. Providing it makes the test fail on Linux.
Since this command is deprecated anyway, we just skip macOS.

crates/radicle-cli/tests/commands.rs

@@ -1689,6 +1689,13 @@ fn rad_fork() {
 
 #[test]
 fn rad_diff() {
+    if std::env::consts::OS == "macos" {
+        // macOS's `sed` requires an argument for `-i`, which we don't provide
+        // in the example. Providing it makes the test fail on Linux.
+        // Since this command is deprecated anyway, we just skip macOS.
+        return;
+    }
+
     let tmp = tempfile::tempdir().unwrap();
 
     fixtures::repository(&tmp);