Support running bevy_lint_driver without the rustc path

[BD103]

Problem

Right now, bevy_lint_driver expects itself to be called like:

$ bevy_lint_driver /path/to/rustc ...ARGUMENTS

It expects its first argument to be the path to rustc, as that's what Cargo passes to it. This, however, can lead to confusion when a normal user tries to run bevy_lint_driver as if it were rustc:

$ rustc --version
rustc 1.89.0-nightly (414482f6a 2025-05-13)

$ bevy_lint_driver --version
Usage: rustc [OPTIONS] INPUT

Options:
    -h, --help          Display this message
        --cfg <SPEC>    Configure the compilation environment.
                        SPEC supports the syntax `<NAME>[="<VALUE>"]`.
# ...

Unbeknownst to them, bevy_lint_driver silently ate --version because it thought it was the path to rustc. Instead, the user should have run:

$ bevy_lint_driver rustc --version

This behavior is problematic because it's undocumented and breaks our UI tests, forcing us to use a workaround.

Solution

The path to rustc is now optional. If it is not specified, bevy_lint_driver assume it is simply the string "rustc" and fill it in for you. This means this now works:

$ bevy_lint_driver --version
rustc 1.89.0-nightly (414482f6a 2025-05-13)

I use some optimized slice trickery to avoid reallocations when performing this adjustment, so the implementation isn't as simple as described. With this change, we can finally remove the ui_test hack, which is super nice!

I still wouldn't recommend using bevy_lint_driver directly as a user, which is why it's not documented much, but this removes one of its sharp edges.

[BD103]

I'm not sure why the UI tests are failing, let me take a look at that . _.

[BD103]

I'm marking this as blocked until v0.4.0 is released.