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

[mboes]

As a followup from #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