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).
We can re-enable these once we're nearing a RIIR release (or if someone thinks
it's a good use of their time to fix them before then). Otherwise we're just
going to have GitHub reporting CI failure for all commits instead of just the
ones that actually broke something.
(I'm mainly trying to get the branch in a good state to merge into master.)
Use rustup to install the latest version of rust. The latest version of rust
available from pkg is 1.66.0 while the code currently needs 1.67.0 or later.
