Auto merge of #84750 - jyn514:nix-cargo, r=Mark-Simulacrum
Auto merge of #84750 - jyn514:nix-cargo, r=Mark-Simulacrum

Don't download cargo twice when download-rustc is set

Previously, this caused a bug on NixOS:

1. bootstrap.py would download and patch stage0/cargo
2. bootstrap.py would download nightly cargo, but extract it to
   stage0/cargo instead of ci-rustc/cargo. It would still try (and fail) to patch ci-rustc/cargo.
3. bootstrap.py would fail to build rustbuild because stage0/cargo
   wasn't patched.

The "proper" fix is to extract nightly cargo to ci-rustc instead, but it
doesn't seem to be necessary at all, so this just skips downloading it
instead.

Fixes https://github.com/rust-lang/rust/issues/84702

aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust