```nbgrader validate``` exit codes when run by a student

### Operating system

macOS Monterrey

### `nbgrader --version`

Python version 3.12.4 | packaged by conda-forge | (main, Jun 17 2024, 10:13:44) [Clang 16.0.6 ]
nbgrader version 0.8.5

### `jupyterhub --version` (if used with JupyterHub)

### `jupyter notebook --version`

6.5.4

### Expected behavior

```nbgrader validate ;print $? ``` should return (print here) a non-zero exit code if it fails any test

### Actual behavior

The command returns 0 even when validation fails

### Steps to reproduce the behavior

1. Create any NBGrader notebook with tests.
2. Release the notebook
3. run ```nbgrader validate ;print $? ```

For context, I'm trying to use nbgrader together with Github Classroom to run my course without having to configure a machine to receive nbgrader submissions, and allow students to get immediate feedback to their work when they commit.