feat(nix): nix and back again

I tried Nix, but it had too many downsides so I removed it.

1. Didn't like that all files are immutable and simple config changes
need a complete rebuild.
2. Setting up a new Mac didn't work as smoothly as promised. Not worth
the effort.
3. It sucked a lot to always have to type in the password twice on each
darwin-rebuild
4. It solves problems I never had.

bin/homebrew-backup

@@ -0,0 +1,2 @@
+#!/bin/bash
+brew bundle dump --force --describe --file=~/.dotfiles/homebrew/$(hostname)