Don't use cargo +TOOLCHAIN shortcut and move --cfg bevy_lint logic

[BD103]

The bevy_lint executable currently runs:

RUSTC_WORKSPACE_WRAPPER=path/to/bevy_lint_driver cargo +TOOLCHAIN check --cfg bevy_lint

This broke for someone on Discord, however, because the +TOOLCHAIN syntax requires Rustup's cargo proxy, which may not be in their PATH. This PR fixes this by instead calling rustup run directly:

RUSTC_WORKSPACE_WRAPPER=path/to/bevy_lint_driver rustup run TOOLCHAIN cargo check --cfg bevy_lint

Both versions do the exact same thing, and both will fail if you do not have Rustup installed, but the latter version should work in a few more scenarios. (And it makes the error message clearer, since it mentions Rustup.)

Furthermore, this PR moves the --cfg bevy_lint logic from the bevy_lint executable to bevy_lint_driver. While the linter should still behave the same, this means programs will still be able to detect the linter even if the use called bevy_lint_driver directly instead of bevy_lint.¹

Footnotes

1. For example, our UI tests call bevy_lint_driver directly. This means we can now technically use #[cfg(bevy_lint)] in our UI tests, even if it doesn't make sense to do so. ↩