alpine docker configuration fails rust tests

[Richard Henderson <rth7680@gmail.com>]

Hiya,

Our Alpine docker image isn't quite right, and fails testing for rust:

> Detecting compiler via: `rustc --target x86_64-unknown-linux-musl --version` -> 0
> stdout:
> rustc 1.83.0 (90b35a623 2024-11-26) (Alpine Linux 1.83.0-r0)
> -----------
> -----------
> Called: `rustc --target x86_64-unknown-linux-musl -C linker=cc -C link-arg=-m64 -o 
> /tmp/qemu-test/build/meson-private/rusttest.exe 
> /tmp/qemu-test/build/meson-private/sanity.rs` -> 1
> stderr:
> error[E0463]: can't find crate for `std`
>   |
>   = note: the `x86_64-unknown-linux-musl` target may not be installed
>   = help: consider downloading the target with `rustup target add x86_64-unknown-linux-musl`
>
> error: aborting due to 1 previous error

But what's even more annoying is that --disable-rust doesn't suppress this check.

Thanks in advance,

r~