admin panel: IT admin can interact with 'use as exit node' but does not have permissions to save

### What is the issue?

On the admin panel's 'edit route settings' dialog, an IT admin is able to see and interact with the 'use as exit node' check box, but upon saving they receive the error `Failed to update route settings`.

According to the [documentation](https://tailscale.com/kb/1103/exit-nodes#allow-the-exit-node-from-the-admin-console), only admins should be able to enable this, so the error is expected (although maybe the error should be about lack of permission for clarity). However, I would expect the option to either be disabled for an IT admin, or not shown at all, to avoid confusion.

### Steps to reproduce

Go to the admin panel as an IT admin, try to allow/disallow 'use exit node' on the route settings dialog. The save should fail.

### Are there any recent changes that introduced the issue?

_No response_

### OS

_No response_

### OS version

_No response_

### Tailscale version

_No response_

### Other software

_No response_

### Bug report

_No response_