bootstrap: Builder::cargo treats rustc_args in ways that don't make sense

[RalfJung]

This was added in #113948:

rust/src/bootstrap/src/core/builder.rs

Lines 2100 to 2105 in bf86c4f

	// set rustc args passed from command line
	let rustc_args =
	self.config.cmd.rustc_args().iter().map(|s| s.to_string()).collect::<Vec<_>>();
	if !rustc_args.is_empty() {
	cargo.env("RUSTFLAGS", &rustc_args.join(" "));
	}

The issue is, this is a Cargo instance, that has a separate rustflags field. So the RUSTFLAGS env var will be overwritten when this is turned into a Command. (Unfortunately that conversion does not have a sanity check that would notice a RUSTFLAGS value getting lost.)

Cc @rust-lang/bootstrap