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

[jyn514]

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 #84702

[rust-highfive]

r? @Mark-Simulacrum

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[jyn514]

bootstrap.py would download nightly cargo, but extract it to
stage0/cargo instead of ci-rustc/cargo.

This happened because of a missing parameter to _download_component_helper, it was missing stage0. Hmm, I think rustfmt is missing that parameter too actually? But rustfmt is working fine for me on nix :/

[jyn514]

Hmm, I think rustfmt is missing that parameter too actually? But rustfmt is working fine for me on nix :/

This is because of the if rustfmt.exists() call:

rust/src/bootstrap/bootstrap.py

Lines 436 to 439 in 49920bc

	if self.rustfmt() and self.rustfmt().startswith(bin_root) and (
	not os.path.exists(self.rustfmt())
	or self.program_out_of_date(self.rustfmt_stamp(), self.rustfmt_channel)
	):

self.rustfmt() is the same regardless of whether stage0 is set, so it just never executes that bit of code when downloading the ci-rustc toolchain, effectively the same behavior as this PR already has.

rust/src/bootstrap/bootstrap.py

Lines 795 to 799 in 49920bc

	def rustfmt(self):
	"""Return config path for rustfmt"""
	if not self.rustfmt_channel:
	return None
	return self.program_config('rustfmt')

[Mark-Simulacrum]

@bors r+

[bors]

📌 Commit 69f3ead has been approved by Mark-Simulacrum

[bors]

⌛ Testing commit 69f3ead with merge 6d4e3c1...

[bors]

☀️ Test successful - checks-actions
Approved by: Mark-Simulacrum
Pushing 6d4e3c1 to master...