don't use rust-arm-cross docker buildx if arch is already arm64

[chrisguida]

This allows fast(er) builds in a Linux VM on MacOS M1. It probably also works on a Pi, though that is untested.

Without this change, the user needs to build rust-arm-cross and then build the backend inside the resulting docker container using buildx. This process is much slower than simply building natively using cargo build (60 minutes -> 16 minutes on my machine).

This change also builds cargo-deps without rust-arm-cross, so rust-arm-cross is removed completely as a system build dependency with this feature, when run on arm64.

Unfortunately the compat image still requires the rust-musl-cross cross-build, as I couldn't figure out a straightforward way of removing that from the build process.

[chrisguida]

Converting to draft, need to fix embassy* and nc-broadcast binaries, there seems to be a version mismatch between glibc versions on my build box and the embassy