Clean up multiarch build setup

[WardBrian]

I am wrestling with this in #1478 and it ended up being the blocker for #1407

The way we currently build the non-x86 linux binaries is a bit cumbersome and a lot brittle.

• We're building them based on Debian images, which at this point are based on the very-out-of-date buster.
• We're building them with --profile=static, which leads to a bunch of warnings about libc, because it's debian.
• The way we select which one to run in build_multiarch_stanc3 is baroque and relies on a package called skopeo -- but I'm 99% sure we could just be passing --platform=linux/arm64 etc instead

I would be nice if we could re-write these such that they were all based on a minimal Alpine image (for static linking), and clean up a lot of the oddities they currently contain