feat: (WIP) Rudimentary, WIP attempt at building risc0's fork of rustc

[mitchmindtree]

Currently failing with the following, haven't dug into this yet:

$ nix build .#risc0-rust
error: builder for '/nix/store/vg35wlql6qlmx5ksz9l2h9b46gkrvn39-risc0-rustc-1.85.0.drv' failed with exit code 2;
       last 25 log lines:
       > Running phase: buildPhase
       > build flags: -j32 SHELL=/nix/store/xy4jjgw87sbgwylm5kn047d9gkbhsr9x-bash-5.2p37/bin/bash
       > warn: Looks like you are trying to bootstrap Rust from a source that is neither a git clone nor distributed tarball.
       > This build may fail due to missing submodules unless you put them in place manually.
       > ERROR: vendoring required, but vendor directory does not exist.
       >        Run `x.py vendor` to initialize the vendor directory.
       >        Alternatively, use the pre-vendored `rustc-src` dist component.
       >        To get a stable/beta/nightly version, download it from:
       >        https://forge.rust-lang.org/infra/other-installation-methods.html#source-code
       >        To get a specific commit version, download it using the below URL,
       >        replacing <commit> with a specific commit checksum:
       >         https://ci-artifacts.rust-lang.org/rustc-builds/<commit>/rustc-nightly-src.tar.xz
       >        Once you have the source downloaded, place the vendor directory
       >        from the archive in the root of the rust project.
       > Traceback (most recent call last):
       >   File "/build/source/src/bootstrap/bootstrap.py", line 1358, in <module>
       >     main()
       >   File "/build/source/src/bootstrap/bootstrap.py", line 1338, in main
       >     bootstrap(args)
       >   File "/build/source/src/bootstrap/bootstrap.py", line 1294, in bootstrap
       >     build.check_vendored_status()
       >   File "/build/source/src/bootstrap/bootstrap.py", line 1200, in check_vendored_status
       >     raise Exception("{} not found".format(vendor_dir))
       > Exception: /build/source/vendor not found
       > make: *** [Makefile:19: all] Error 1
       For full logs, run:
         nix log /nix/store/vg35wlql6qlmx5ksz9l2h9b46gkrvn39-risc0-rustc-1.85.0.drv