hacking.md: add --prefix flag to configure
hacking.md: add --prefix flag to configure

Otherwise, the steps advertised in this document won't actually work
(e.g. `make install` will fail, trying to access /usr, and
`./inst/bin/nix` won't exist).

rycee/nix,NixOS/nix,rycee/nix,rycee/nix,NixOS/nix,NixOS/nix,NixOS/nix,rycee/nix,rycee/nix,NixOS/nix