If `nix-shell` is run interactively on NixOS, in way that has loaded
`/etc/profile` (for example `docker exec <container> nix-shell`), then
`/etc/profile` will be sourced in the shell that nix-shell starts. That
overwrites the `NIX_PATH` set by `shell.nix`.

If we instead set `NIX_PATH` in the the `shellHook`, that always takes
precedence.

no reason to keep carrying out custom graphviz package or build all this old
junk just for the docs

no reason to keep carrying out custom graphviz package or build all this old
junk just for the docs

The nixpkgs fork already contains all of the changes our overlays were
responsible for - but as direct nixpkgs changes instead.

this means we don't have to download it every time

Maybe I should be wasting my time learning about Hydra instead ...