Change nannou-new to use installed instance of cargo rather than the cargo crate

[mitchmindtree]

This would greatly improve nannou-new compile times and also avoid an issue that sometimes occurs where the package needs to be built twice as a result of the separate cargos (the first build by the cargo dep, the second in the user's environment).

It would be nice to address this before #604.

[kneitinger]

Hi @mitchmindtree I'm beginning to work on this (as well as #448), and have one clarifying question. Is the point of this task to call the cargo as a std::process::Command, or to somehow use a system-wide copy of the cargo crate? Nothing in my investigation was able to suggest that the latter is something that exists, so I just wanted to verify that a Command based approach was intended before embarking on that. Thanks!

[mitchmindtree]

Hi @kneitinger!

Is the point of this task to call the cargo as a std::process::Command, or to somehow use a system-wide copy of the cargo crate?

Yes, kind of both. Like you, I was unable to find a nice library for using the user's pre-installed cargo instance from a library in a nice manner, but for such a small task I agree using Command is probably easiest :)