Draft: Trim down the QEMU build

Refs #88

Trying to slim down our deployed software.

This MR builds on a suggestion by @jcalderone in #88. I don't think I am doing it right though, since the reported difference is minimal:

$ nix --extra-experimental-features nix-command store diff-closures result-before/ result-after/

initrd-linux: +13.1 KiB