Update our MSRV to Rust 1.85.
Includes fixes for lints which were previously suppressed due to them
relying on features added after Rust 1.70.
Rust 1.85 prints a warning when using `#[cfg(target_os = "cygwin")]`, so
we work around the one instance where this is a problem for now. This
workaround can be reverted when we update to Rust 1.86 or newer.
Certain old versions of macOS are no longer supported by Rust starting
with Rust 1.74, so this commit raises our macOS version requirement to
10.12.
https://blog.rust-lang.org/2023/09/25/Increasing-Apple-Version-Requirements/https://github.com/fish-shell/fish-shell/pull/11961#discussion_r2442415411Closes#11961
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).
