Fix RUSTUP_PERMIT_COPY_RENAME condition so it is actually used

[dylanahsmith]

cc @mckaymatt

Problem

It looks like #2410 stopped working because it has a match on the io::ErrorKind::Other, but the experimental io::ErrorKind::CrossesDevices variant was later added, which prevented it from every matching the match arm for the RUSTUP_PERMIT_COPY_RENAME condition. As such, it would never fallback to copying and deleting and just continue failing with the "Invalid cross-device link" error.

Solution

An explicit match on io::ErrorKind::Other wasn't necessary (it already is conditional on the raw_os_error) and this bug showed how matching on it makes the code fragile to the addition to new variants, so I just replaced it with a _ catch all match with the same condition.

Testing

I've managed to manual test this in isolation, although the use of sudo to mount an overlayfs seems like it would be a problem with testing this through cargo test.

# dev setup as suggested in CONTRIBUTING.md
mkdir home
export RUSTUP_HOME=home CARGO_HOME=home
target/debug/rustup-init --no-modify-path --default-toolchain=none -y

# force download of an older version to test upgrading
home/bin/rustup toolchain install 1.68.1
mv home/toolchains/{1.68.1-x86_64-unknown-linux-gnu,stable-x86_64-unknown-linux-gnu}

# turn rustup home into an overlay over the initial rustup installation
mv home homelower
mkdir homeupper homework home
sudo mount -t overlay overlay -o lowerdir=homelower,upperdir=homeupper,workdir=homework home

RUSTUP_PERMIT_COPY_RENAME=1 home/bin/rustup update

# cleanup
sudo umount home
rm -r homework home homelower homeupper

On master, I get the "could not rename component file" error, but it works when retrying the above steps on this branch.

[rbtcollins]

@hi-rustin letting you know I'm merging this in - its a regression fix, and super simple