Linter: Support for running cargo check without toolchain proxy in non-rustup environments

[DaAlbrecht]

Currently the build system for bevy_lint enforces a specific toolchain proxy via the +<channel> syntax in the cargo check command, assuming the use of rustup. This is incompatible with setups where cargo is installed outside of rustup for example with Nix.

[BD103]

Does Nix allow installing specific nightly toolchains and components? If not, we may not be able to support it.

[Multirious]

Does Nix allow installing specific nightly toolchains and components? If not, we may not be able to support it.

Yep, it allows. When packaging and installing in Nix, we can declare a specific toolchain version as dependency.

• Simplest way is probably to just add a config/env to not pass +... to cargo and just select the current cargo in PATH.
• Config/env var for explicit cargo location at runtime.
• Config/env var for explicit cargo location at compile time.

Oh, I just remembered that the driver has to be compiled specifically for a single nightly version. An env variable at compile time is probably the only useful option here.

[BD103]

We could also add a feature flag, such as --feature nix, so the user can specify explicitly that they are not using Rustup.