Nix: warn when pkg-config or git not present in nix-shell

As a followup from https://github.com/commercialhaskell/stack/issues/2089, we should be helpful to the user and suggest fixes whenever we can. In particular, it's confusing that a `stack exec` command might fail because `git`, `pkg-config` or some other tool is not present, when it's most likely installed in the user's host environment.

In particular, if `--nix` is passed an the `stack` subprocess fails to find the git command, Stack should suggest adding it to the buildInputs. Likewise for `pkg-config`. There's really not much else that Stack would need for building that isn't app specific.

cc @YPares @rrnewton