Fix #6861 Use nix-instantiate command, with failure exception

[mpilgrem]

See:

• If nix-shell <args> fails, Stack just outputs the Nix error #6861

• Any changes that could be relevant to users have been recorded in ChangeLog.md.

• The documentation has been updated, if necessary

Please also shortly describe how you tested your change. Bonus points for added tests!

If any users of Stack on NixOS or other users of Stack's Nix integration see this, I would appreciate any feedback on it.

[mpilgrem]

I have converted to draft, as there is unexplained adverse changed behaviour in respect of locale when nix-shell is used with a store derivation of an expression rather than with the expression directly. I'll document that more fully later.