add nix.packages to stack.yaml
add nix.packages to stack.yaml

postgrest depends on postgres and zlib. When using stack's nix
integration, these dependencies must be specified explicitly.

Note that `--nix` must be passed to stack, or `nix.enable:true` must
be set in `stack.yaml` to enable using nix while building.

begriffs/postgrest,steve-chavez/postgrest,Skyfold/postgrest,steve-chavez/postgrest,ruslantalpa/postgrest,diogob/postgrest