set `CARGO` instead of `PATH`

[onur-ozkan]

Previously, clippy was using cargo from PATH, but since rust-lang/rust-clippy#11944, it now prioritizes checking CARGO first. We can now set cargo path into CARGO and avoid appending it to the PATH chain.

In the bootstrap, we have this piece of code

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

Lines 1220 to 1226 in 7f1d08e

	// Set PATH to include the sysroot bin dir so clippy can find cargo.
	// FIXME: once rust-clippy#11944 lands on beta, set `CARGO` directly instead.
	let path = t!(env::join_paths(
	// The sysroot comes first in PATH to avoid using rustup's cargo.
	std::iter::once(PathBuf::from(initial_sysroot_bin))
	.chain(env::split_paths(&t!(env::var("PATH"))))
	));

which adds cargo into PATH. As indicated by the FIXME note, rust-lang/rust-clippy#11944 wasn't merged at the time. Now that we can do so, set CARGO instead of PATH and remove the FIXME note.

[long-long-float]

@rustbot claim

[RalfJung]

There's more weird stuff going on here...

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

Lines 1251 to 1252 in 4797fba

	cmd.env(helpers::dylib_path_var(), env::join_paths(&dylib_path).unwrap());
	cmd.env("PATH", path);

On Windows, helpers::dylib_path_var() is PATH, so this sets the PATH and then immediately overwrites it again with a different value...?!?

This almost looks like it may want to call add_dylib_path or add_rustc_lib_path. It's not quite equivalent though. Unfortunately there are no comments explaining why it does what it does... Cc @asquared31415 who wrote that code, but it's been two years so not sure if they remember.^^

[asquared31415]

Yep, sorry, absolutely no clue what that code is meant to do. It's very likely that I copied that from somewhere else during the initial stages of "try everything and see if any of it works", or copied some part of that, and eventually modified to set PATH. I have also unfortunately forgotten all of the context of the bootstrap code here, how it's meant to flow, and what it's meant to set for other programs, so I will not be of any help here :c

I will try to check my messages with jyn, who i was talking to a lot about this at the time, and trying many things at their suggestion, to see if I can find any useful history.