fix check rustup on default toolchain

[fraidev]

Seems that any cross command is trying to add a rustup toolchain even if is already installed.

I checked that was just checking the rustup --quiet toolchain list command and comparing each one, but this check will not work for the active one with (active, default).

Example:

▷ rustup --quiet toolchain list
stable-aarch64-apple-darwin (active, default)
stable-x86_64-unknown-linux-gnu
nightly-aarch64-apple-darwin
nightly-2024-10-31-aarch64-apple-darwin
1.72.0-aarch64-apple-darwin
1.74.0-aarch64-apple-darwin
1.75.0-aarch64-apple-darwin
1.78.0-aarch64-apple-darwin
1.79.0-aarch64-apple-darwin
1.80.0-aarch64-apple-darwin
1.81.0-aarch64-apple-darwin
1.82.0-aarch64-apple-darwin
1.83.0-aarch64-apple-darwin
1.84.0-aarch64-apple-darwin
1.85.0-aarch64-apple-darwin
1.86.0-aarch64-apple-darwin

I needed this fix because setuptools-rust calls cargo/cross metadata and convert it to json, but I was always getting it before the json expected:

stable-x86_64-unknown-linux-gnu unchanged - rustc 1.87.0 (17067e9ac 2025-05-09)

[reneleonhardt]

@fraidev doesn't #1681 solve your problem?

rustup toolchain list --quiet