Fix CI failures

[Nemo157]

Remove -Z install-upgrade for cargo-hack, this is now the default since rust-lang/cargo#7560.

Add allow(unreachable_code) for usage of try_join with the never type, fixes #1992.

[Nemo157]

Although, given the latest conclusions in rust-lang/rust#66757 that test will start failing to compile on a future nightly. It can probably use core::convert::Infallible which will not actually test what it intends to test once the never type is reverted, but as soon as it is stabilised again the test will be correct and let us know if there's been a regression, and the fix works fine either way.