It appears we can't find a system that ships rustc >= 1.67 and < 1.70, so keeping it at 1.67 gains nothing. 1.70 is used in Debian 13, so that will be able to build fish out of the box (12 was on 1.63 which was already too low).
* Add machine-readable MSRV to Cargo.toml * Fix clippy warnings * CI: add rustfmt and clippy checks
Closes #9556