Rust and cargo manifests need updating

[analogrelay]

There are some issues with the rust-nightly and cargo-nightly manifests. Because they generate shim exes, the cargo build tool has trouble shelling out to the rustc compiler (see rust-lang/cargo#1661). Also, cargo is no longer a separate component, it is generally packaged together with rust so having a separate manifest for it is confusing.

I'll be submitting a PR for this momentarily. It's fixed the issue in my environment, but I'll loop in the original submitters of the {rust,cargo}-nightly manifest so they can take a look too.

Scoop-Extras should (and my PR will):

1. Remove the cargo-nightly manifest, since rustc and cargo are generally installed together from a single MSI
2. Change from using shims to env_add_path because otherwise Cargo not properly escaping quotes in Windows command lines rust-lang/cargo#1661 gets in the way. I'd still like to track down why the shim isn't working, since I prefer the shim mechanism, but for now this will fix the issue.

[lukesampson]

Thanks for investigating this Andrew. I wonder if the shim could be fixed by switching shim.cs to use Environment.CommandLine instead of the parsed arguments.