cargo: increase rust-version to 1.89

[xtqqczze]

https://ubuntu.com/developers/docs/reference/availability/rust/

[cakebaker]

Hm, how do you get to 1.89 from that list?

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/cut/bounded-memory (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/seq/seq-epipe is now being skipped but was previously passing.

[xtqqczze]

There's no reason to go higher than 1.89 at present, we don't have any dependencies that need a higher version.

[oech3]

Ubuntu LTS itself does not bump version of rust-coreutils. So there is no reason to align with LTS (aligning with 26.10 might making some sense).

We should assume that devs on LTS are using latest Rust via rustup.