Fix diagnostics variable name in add_plugin.py script (astral-sh#…

…5317)

## Summary

Fix a variable name in the `add_plugin.py` script.

## Test Plan

I don't think there are any tests for the scripts, other than manual
confirmation

scripts/add_plugin.py

@@ -45,7 +45,7 @@ def main(*, plugin: str, url: str, prefix_code: str) -> None:
 
     fn rules(rule_code: Rule, path: &Path) -> Result<()> {
         let snapshot = format!("{}_{}", rule_code.as_ref(), path.to_string_lossy());
-        let messages = test_path(
+        let diagnostics = test_path(
             Path::new("%s").join(path).as_path(),
             &settings::Settings::for_rule(rule_code),
         )?;