Remove dependency on Rustup for `bevy_lint`

[BD103]

Our NixOS friends don't use Rustup, so they currently have no way to use the linter! We may be able to get around this, but we'll have to find a replacement that works without Rustup for the following lines:

bevy_cli/bevy_lint/src/bin/main.rs

Lines 25 to 43 in fa01e7e

	let status = Command::new("rustup")
	.arg("run")
	.arg(RUST_TOOLCHAIN_CHANNEL)
	.arg("cargo")
	.arg("check")
	// Forward all arguments to `cargo check` except for the first, which is the path to the
	// current executable.
	.args(std::env::args().skip(1))
	// This instructs `rustc` to call `bevy_lint_driver` instead of its default routine.
	// This lets us register custom lints.
	.env("RUSTC_WORKSPACE_WRAPPER", driver_path)
	// Rustup on Windows does not modify the `PATH` variable by default so a toolchain-specific
	// version of `cargo` or `rustc` is not accidentally run instead of Rustup's proxy version.
	// This isn't desired for us, however, because we need the `PATH` modified to discover and
	// link to `rustc_driver.dll`. Setting `RUSTUP_WINDOWS_PATH_ADD_BIN=1` forces Rustup to
	// modify the path. For more info, please see <https://github.com/rust-lang/rustup/pull/3703>.
	.env("RUSTUP_WINDOWS_PATH_ADD_BIN", "1")
	.status()
	.context("Failed to spawn `cargo check`.")?;

[BD103]

Related links:

• PR requiring us to set RUSTUP_WINDOWS_PATH_ADD_BIN: Change default for RUSTUP_WINDOWS_PATH_ADD_BIN rust-lang/rustup#3703
  • Append Windows "bin" directory to PATH rust-lang/rustup#4249 means we no longer need to set it for modern Rustup installations, but we keep it in case users have an older version of Rustup.
• How to cargo-nextest avoids using Rustup: Document RUSTUP_WINDOWS_PATH_ADD_BIN rust-lang/rustup#4196 (comment)

I think the best course of action is to run rustc --print target-libdir and append its results to LD_LIBRARY_PATH (Linux) / PATH (Windows) / DYLD_LIBRARY_PATH (MacOS). That should remove our need to depend on Rustup, as the only other unique part (RUSTC_WORKSPACE_WRAPPER) is for Cargo.

In order to figure out which rustc is run, I think we should support a BEVY_LINT_RUSTC environmental variable that points to the path to rustc. If it is set, we use that path. If it is not set, we run rustup which rustc --toolchain nightly-2025-04-22 and use that's returned path.