fix: Symlink rustfmt instead of copying

[emgrav]

This should solve an issue I was having on aarch64-darwin where the rustfmt binary is apparently dynamically linked.

[cursor]

PR Summary

Low Risk
Small Nix dev-toolchain packaging change with no runtime app or security impact.

Overview
Updates famedly-rust-toolchain packaging so nightly cargo-fmt and rustfmt are symlinked from the nightly toolchain instead of copied into the composite toolchain’s bin/.

That keeps rustfmt working on aarch64-darwin, where those binaries are dynamically linked and need the original toolchain’s lib/ layout. A short comment documents the platform-specific reason.

^{Reviewed by Cursor Bugbot for commit 27efecb. Bugbot is set up for automated code reviews on this repo. Configure here.}

[tlater-famedly]

LGTM, in spite of my preference on how this file is commented.

[tlater-famedly]

Nit: We should keep this comment in addition to the new one. This comment explains why overriding parts of the toolchain like this is fine, the other one explains why we're symlinking instead of copying.

[emgrav]

Good point, thanks. Done