With rust-lang/cargo#13659, cargo now respects RUSTC_WRAPPER for all rustc invocations. So bootstrap should be able to set RUSTC to a non-existing binary and rely on RUSTC_WRAPPER for everything, which could simplift some logic.

See #127682 for what that could look like. However, this is currently blocked on rust-lang/libc#3772, and there are likely other components that also still use RUSTC (e.g. Miri and/or clippy might rely on it when running in x.py).