Fix using local stack snapshots

[kirelagin]

This used to be part of input-output-hk#334, except that I added a test for this and made a little tweaks hear and there to make sure that it actually works 🙄.

The newly added test now passes, but, for some reason, it produced the following warning, I have no clue why:

substituteStream(): WARNING: pattern '/nix/store/<hash>-stack-local-resolver' doesn't match anything in file '/nix/store/<hash>-stack-to-nix-pkgs/stack-local-resolver.nix'
substituteStream(): WARNING: pattern '/nix/store/<hash>-stack-local-resolver' doesn't match anything in file '/nix/store/<hash>-stack-to-nix-pkgs/default.nix'
substituteStream(): WARNING: pattern '/nix/store/<hash>-stack-local-resolver' doesn't match anything in file '/nix/store/<hash.-stack-to-nix-pkgs/pkgs.nix'

[kirelagin]

wait what