Disallow `rev` for builtin repos in schema

```
error: Failed to parse `prek.toml`
  caused by: TOML parse error at line 1, column 1
  |
1 | [[repos]]
  | ^^^^^^^^^
`rev` is not allowed for builtin repos
```

`rev` can't be used for builtin repos, but the schema allows this. It seems to be pretty strict otherwise, so this should be disallowed in the schema.