Perform install with a script rather than forced archive extraction if available.

[RoboDoig]

The Unity installer currently extracts the Bonsai archive and performs a 'first run' process. The issue with this is that it can overwrite any existing config files left in a repo.

We should instead follow the practice of other Bonsai environments and defer to a powershell script if it exists in the Bonsai directory of a Unity project.