bundle/cargo: respect CARGO_HOME and friends

[branchv]

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Similar to #20866 for go, this passes through cargo env variables that affect cargo install --list and cargo install

• CARGO_HOME and CARGO_INSTALL_ROOT tell cargo where to put binaries
• RUSTUP_HOME tells a rustup-provided cargo where to find its default toolchain (which will otherwise fail with rustup could not choose a version of cargo to run)

[Copilot]

Pull request overview

This PR updates Homebrew Bundle’s Cargo integration to preserve user Cargo/Rustup environment configuration across Homebrew’s environment filtering, matching the existing approach used for other toolchains (e.g., Go).

Changes:

• Pass through CARGO_HOME, CARGO_INSTALL_ROOT, and RUSTUP_HOME by exporting them as HOMEBREW_* variables in bin/brew.
• Apply those variables when running cargo install --list and cargo install in the Bundle Cargo extension via a shared cargo_env helper.

Reviewed changes

Copilot reviewed 1 out of 2 changed files in this pull request and generated 1 comment.

File	Description
bin/brew	Allows Cargo/Rustup-related env vars to survive env -i filtering by copying them to HOMEBREW_*.
Library/Homebrew/bundle/extensions/cargo.rb	Wraps Cargo invocations with with_env(cargo_env(...)) so Cargo sees the intended install/home/toolchain dirs.

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[Copilot]

The new env pass-through logic (CARGO_HOME/CARGO_INSTALL_ROOT/RUSTUP_HOME via cargo_env and with_env) isn’t covered by tests. Since this file already has specs, please add assertions that these variables are set/passed through when corresponding HOMEBREW_* vars are present (and that they don’t leak outside the block).

[carlocab]

Suggested change

	sig { params(cargo: Pathname).returns(T::Hash[String, T.nilable(String)]) }
	def cargo_env(cargo)
	{
	"CARGO_HOME" => ENV.fetch("HOMEBREW_CARGO_HOME", nil),
	"CARGO_INSTALL_ROOT" => ENV.fetch("HOMEBREW_CARGO_INSTALL_ROOT", nil),
	"PATH" => "#{cargo.dirname}:#{ENV.fetch("PATH")}",
	"RUSTUP_HOME" => ENV.fetch("HOMEBREW_RUSTUP_HOME", nil),
	}
	sig { params(cargo: Pathname).returns(T::Hash[String, String]) }
	def cargo_env(cargo)
	{
	"CARGO_HOME" => ENV.fetch("HOMEBREW_CARGO_HOME", nil),
	"CARGO_INSTALL_ROOT" => ENV.fetch("HOMEBREW_CARGO_INSTALL_ROOT", nil),
	"PATH" => "#{cargo.dirname}:#{ENV.fetch("PATH")}",
	"RUSTUP_HOME" => ENV.fetch("HOMEBREW_RUSTUP_HOME", nil),
	}.compact

[MikeMcQuaid]

Agreed with comments, otherwise looks good!