generatedCargoNix: allow selected toolchain

[bengsparks]

Rust projects can use newer or custom toolchains that are not available in nixpkgs, even more so now that Rust Edition 2024 is upon us.

Attempting to generate Cargo.nix via IFD for a project with edition 2024 currently fails, no matter which version of cargo would be found on the $PATH first.

Logs from build failure

Running phase: unpackPhase
@nix { "action": "setPhase", "phase": "unpackPhase" }
unpacking source archive /nix/store/hx77ikpfvx0r0l6w7q20gcwh0qass5j7-rust
source root is rust
Running phase: buildPhase
@nix { "action": "setPhase", "phase": "buildPhase" }
++ crate2nix generate -f ./Cargo.toml -o Cargo-generated.nix -h /nix/store/wzj8q30d3kalhpsrhray6vj5y8xazl89-workspace-crate2nix/crate-hashes.json
Error: while retrieving metadata about ./Cargo.toml: `cargo metadata` exited with an error: warning: `/nix/store/wzj8q30d3kalhpsrhray6vj5y8xazl89-workspace-crate2nix/cargo/config` is deprecated in favor of `config.toml`
note: if you need to support cargo 1.38 or earlier, you can symlink `config` to `config.toml`
error: failed to load manifest for workspace member `/private/tmp/nix-build-workspace-crate2nix.drv-0/rust/nix-cst`
referenced by workspace at `/private/tmp/nix-build-workspace-crate2nix.drv-0/rust/Cargo.toml`

Caused by:
  failed to parse manifest at `/private/tmp/nix-build-workspace-crate2nix.drv-0/rust/nix-cst/Cargo.toml`

Caused by:
  feature `edition2024` is required

  The package requires the Cargo feature called `edition2024`, but that feature is not stabilized in this version of Cargo (1.82.0 (8f40fc59f 2024-08-21)).
  Consider adding `cargo-features = ["edition2024"]` to the top of Cargo.toml (above the [package] table) to tell Cargo you are opting in to use this unstable feature.
  See https://doc.rust-lang.org/nightly/cargo/reference/unstable.html#edition-2024 for more information about the status of this feature.

crate2nix failed.
== cargo/config (BEGIN)
    [source.crates-io]
    replace-with = "vendored-sources"
    
    
    [source.vendored-sources]
    directory = "/nix/store/4h8s39mxr5c0im1snn153wam1hym45c4-deps"

== cargo/config (END)

== crate-hashes.json (BEGIN)
    {}
== crate-hashes.json (END)

== ls -la (BEGIN)
total 16
drwxr-xr-x 7 _nixbld1 nixbld 224 Jan  1  1970 .
drwx------ 4 _nixbld1 nixbld 128 Feb 21 23:13 ..
-rw-r--r-- 1 _nixbld1 nixbld 437 Jan  1  1970 .gitignore
-rw-r--r-- 1 _nixbld1 nixbld 151 Jan  1  1970 Cargo.lock
-rw-r--r-- 1 _nixbld1 nixbld  49 Jan  1  1970 Cargo.toml
-rw-r--r-- 1 _nixbld1 nixbld 325 Jan  1  1970 default.nix
drwxr-xr-x 4 _nixbld1 nixbld 128 Jan  1  1970 nix-cst
== ls -la (END)

The reason for this is that crate2nix uses pkgs.cargo by default, and pkgs is already bound to what flake.nix is locked to, which yields a Cargo 1.82 currently, which does not support edition 2024.

This CAN be overridden as per this guide, which requires setting cargo in an overlay prior thereto.

While functional, this approach is quite blunt, requiring a flake-wide override just to satisfy a tool that is not used to build the actual project.
My proposed solution is to simply pass a cargo argument to generatedCargoNix function. If it is omitted, then we fallback to pkgs.cargo, thereby retaining existing behavior.
This is particularly handy in combination with rust-overlay, and likely fenix (I've only tested this change with the former).