fix: Plug leak in EvalState::realise_string

(cherry picked from commit d9f9aa64ceb48c39c028d48680a664063d70bc23)

rust/nix-expr/src/eval_state.rs

@@ -175,6 +175,11 @@ impl EvalState {
             paths
         };
 
+        // We've converted the nix_realised_string to a native struct containing copies, so we can free it now.
+        unsafe {
+            raw::realised_string_free(rs);
+        }
+
         Ok(RealisedString { s, paths })
     }