Moves the nix script to setup the build environment from Documentation
into the Toolchain as a callable script. I also modified the script
to accept a "pkgs" argument to make it easy to override the nixpkgs
version from the command-line when calling the script.
I was building serenity on quite a fresh NixOS system and it turns
out `unzip` and `qemu` were missing from this nix expression to
compile and run serenity.
This removes the '$' character so that it is easier to copy commands
directly from the build instructions and then executing them without
first having to remove the '$' character.
