Before, with services.jupyter.enable set but no user specified, user creation
failed with:
```
error:
Failed assertions:
- Exactly one of users.users.jupyter.isSystemUser and users.users.jupyter.isNormalUser must be set.
```
Set isSystemUser to true when creating the jupyter user.
* Fix documentation example for `jupyter.kernels`
The environment variable loading fails when using the example for `kernels` config, due to incorrect syntax. The error being something along the lines of `path not found`.
Thanks to @Infinisil and @layus for suggestions.
