Testing on user and system installs

At least for Mac, for testing, we currently do a system install (install for all users) - see https://github.com/scientific-python/installer/blob/main/.github/workflows/build.yml#L151:

```
sudo installer -verbose -pkg ${MNE_INSTALLER_NAME} -target / 
```

We might want to test user installs as well:

```
installer -verbose -pkg ${MNE_INSTALLER_NAME} -target CurrentUserHomeDirectory
```

