ci: update pinned to Rust 1.88

[mathstuf]

Required by new time crates (time, time-core, time-macros).

[arckoor]

Thanks, but ultimately this is a MSRV problem, you can't just bump CI. Root cause is that simple_asn1 pinned their time dependency recently, which forces that MSRV onto us.
I've asked whether that was really necessary here (if someone sees something I don't in regards to their rfc2822 usage, please chime in).
Whatever the solution turns out to be, I'll likely address it in #498.

[mathstuf]

Ok. It's just that with the way GHA works, this means that PR pipelines get no useful information until this is fixed. I understand why they cancel all jobs when any fails (don't waste (free) compute time), but it can mean more duplicate work to tackle problems one-by-one in the long run (as you run jobs over and over to just get "the next problem" instead of being able to do a single update to tackle a lot of uncovered issues).