Auto merge of #96687 - jyn514:download-rustc, r=Mark-Simulacrum

Move download-rustc from python to rustbuild

- Remove download-rustc handling from bootstrap.py
- Allow a custom `pattern` in `builder.unpack()`
- Only download rustc once another part of bootstrap depends on it.

  This is somewhat necessary since the download functions rely on having a full
  `Builder`, which isn't available until after config parsing finishes.

Helps with https://github.com/rust-lang/rust/issues/94829.