Build riscv toolchains from source

Currently we download a prebuilt version and patch it up. We probably want to build it from source with nix derivation directly.

Start by locating the derivation that downloads and patches the prebuilt RISC-V toolchain, then trace how the resulting toolchain is exposed by the package. Confirm the source-build inputs and derivation behavior before changing it; done means the toolchain builds directly from source without the current prebuilt download-and-patch step.

Written by the indexing model from the issue text.