How to add a custom toolchain

[eureka-cpu]

Hello, sort of running into a similar problem as what I described some time ago in #110 so I'm thinking of implementing a work around for fenix, but first I need to understand how a custom toolchain would be created. I've seen a few suggestions of using toolchainOf, but how exactly would I use that for a toolchain like solana which is derived from https://github.com/solana-labs/solana and is necessary for running cargo-build-sbf which uses rustup under the hood to manage the cargo invocation to use their custom toolchain? I have the solana packages building with nix, and if I add rustup to my buildInputs the toolchain solana is there:

$ rustup toolchain list
solana

which indicates to me that somewhere the components for the toolchain are available in my build. Is it possible to access those components and create the solana toolchain using toolchainOf or some other fenix function?

[eureka-cpu]

I think I see maybe two options:

1. find some way to make rustup aware of the fenix toolchain, and just use rustup as it's currently used (probably easiest if possible)
2. assuming fenix can have multiple toolchains at once, package the solana toolchain, add it to the current toolchain and patch the solana rust code so that it doesn't use rustup but instead uses fenix to handle the cargo toolchain invocations

[eureka-cpu]

@figsoda if I was to for the first option pretty much all I need to know is where the toolchain is located and I think I can use rustup toolchain link <name> <path>. Do you think this would work?