rust 1.91 incoming

[Thomas Klausner <wiz%netbsd.org@localhost>]

Hi!

I've run a bulk build of bulk-test-rust with wip/rust191 (packaged by
he@, thank you!) on NetBSD 11.99.4/x86_64.

There was no breakage.

All bootstrap kits in the rust Makefile have been updated to 1.90,
including Solaris.

I'll commit the update now.

Please check if wip/rust192 builds fine on your favorite platform (but
I have no plans to update to this version quickly, since I think we'll
need a newer llvm for that).

Cheers,
 Thomas