[CI] Rust beta toolchain

[Mirko-von-Leipzig]

The 2024 State of Rust survey has a call to increase feedback on the beta toolchain:

Unsurprisingly, the most popular version of Rust is latest stable, either the most recent one or whichever comes with the users' Linux distribution. Almost a third of users also use the latest nightly release, due to various reasons (see below). However, it seems that the beta toolchain is not used much, which is a bit unfortunate. We would like to encourage Rust users to use the beta toolchain more (e.g. in CI environments) to help test soon-to-be stabilized versions of Rust.

Perhaps we could add a cron trigger for our CI workflow to run against the beta toolchain, e.g. that runs once a day against current next. This doesn't cost us anything besides setting it up, and investigating failures if we feel like it :)