Decouple from Linuxbrew due to lack of ARM support

[eschaton]

On macOS, podman creates native rather than x86-64 containers, so podman build -t roscopeco/rosco_m68k . won't actually complete because, as it says in the error output, Homebrew on Linux is not supported on ARM processors.

If the toolchain could be set up in a way that will Just Work atop the Debian image it uses as a base—or directly atop the Fedora CoreOS that podman uses—then it would probably work natively in many more environments.