fix: use toWTFString for JSValue string conversion in initgroups

- Replace user.getString(globalObject) with user.toWTFString(globalObject)
- Fix compilation errors: JSString* type mismatch and missing utf8() method
- Proper exception handling with RETURN_IF_EXCEPTION

Co-authored-by: Jarred-Sumner <Jarred-Sumner@users.noreply.github.com>

src/bun.js/bindings/BunProcess.cpp

@@ -2565,9 +2565,9 @@ JSC_DEFINE_HOST_FUNCTION(Process_functioninitgroups, (JSGlobalObject * globalObj
         }
         username = pp->pw_name;
     } else {
-        JSString* str = user.getString(globalObject);
+        auto userString = user.toWTFString(globalObject);
         RETURN_IF_EXCEPTION(scope, {});
-        auto utf8 = str->utf8();
+        auto utf8 = userString.utf8();
         username = utf8.data();
     }