Rust nightly toolchain (rustc, cargo, rust-std and rust-src) pinned to
the version required by lang/bun and installed from the official
upstream binary distribution.

This port exists because lang/bun requires a specific Rust nightly:
bun's source uses unstable language and library features, and its
release build passes -Zbuild-std, which needs the rust-src component.
Depending on lang/rust-nightly makes lang/bun break whenever that port
is updated, so lang/bun depends on this pinned toolchain instead.

The toolchain is installed under PREFIX/rust-nightly-bun and does not
conflict with lang/rust or lang/rust-nightly.