Set `NIX_PATH` in `shellHook` of `shell.nix`.
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.