Faster Nix builds with GitHub Actions and actuated

A note from my colleague Han that wrote this article

> I often deploy faasd with NixOS on a Raspberry Pi and to the cloud, so I build binaries for both x86_64 and aarch64. The build usually runs on the default GitHub hosted action runners. Now because GitHub currently doesn't have Arm support, I use QEMU instead which can emulate them. The drawback of this approach is that builds can sometimes be several times slower.