Change Nushell activation script to module

[kubouch]

Description

The activation script will be a module that would be added as an overlay which allows for more ergonomic handling of the environment varaibles.

I had to change the source activation command to overlay use because that's how you activate overlay in Nushell. However, it required me to also disable quoting of the command. By default, the Python activator would create 'overlay use' and try to call that as a command but in Nushell, commands are called without quotes.

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