Failing to build cross-gem needing latest Rust version/edition

[cretz]

I have a cross-gem run that is failing with:

error[E0658]: `let` expressions in this position are unstable
   --> ext/sdk-core/core-api/src/worker.rs:226:12
    |
226 |         if let Some(Some(ref x)) = self.max_worker_activities_per_second
    |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: see issue #53667 <https://github.com/rust-lang/rust/issues/53667> for more information

A regular bundle exec rake compile works just fine. Can see the PR (and failing checks) at temporalio/sdk-ruby#305 where I have tried various things to fix this including setting edition to 2024 and trying to use a .cargo/config.toml file and setting pre-script: rustup update stable as an option. I can re-run the failing CI checks if the logs are no longer available as of this writing.

What is the best way to make sure that cross-gem action uses latest stable Rust? Is this supported and/or am I doing something incorrect?

[cretz]

Note, we worked around this by using rb-sys-dock directly so we could pass --mount-toolchains. The fix for this is probably just to support that as an action input.