Update build.rs

Update comment about why both file lock and mutex are needed.

tooling/nargo_cli/build.rs

@@ -168,10 +168,11 @@ fn generate_test_cases(
     }
     let test_cases = test_cases.join("\n");
 
-    // Locking the Nargo.toml file in the test directory so that we can run tests exclusively.
-    // NB a Mutex is not enough on CI because we use `cargo nextest` which can run test cases
-    // in the group in different processes.
-    // For some reason the file lock alone doesn't seem to do the trick locally.
+    // We need to isolate test cases in the same group, otherwise they overwrite each other's artifacts.
+    // On CI we use `cargo nextest`, which runs tests in different processes; for this we use a file lock. 
+    // Locally we might be using `cargo test`, which run tests in the same process; in this case the file lock
+    // wouldn't work, becuase the process itself has the lock, and it looks like it can have N instances without
+    // any problems; for this reason we also use a `Mutex`.
     let mutex_name = format! {"TEST_MUTEX_{}", test_name.to_uppercase()};
     write!(
         test_file,