Anyone using `hpack` and/or `stack` will have a dependency on `internal-nix-tools` (built with a fixed version of ghc). This is fine if your `nixpkgs` is one of the ones we cache, but if not we may have to wait for an extra ghc to build. When this happens it would be nice if it is at least building a more recent version (rather than an older one).
* Cross comp. & callCabalProjectToNix/callStackToNix
This fixes an issue where the these IFD functions fail because they try
to build and run a cross compiled nix-tools on the build system.
It may also help if there are cross compilation issues with
setup-depends (they also access the buildPackages).
* Align stack-to-nix ifd with plan-to-nix ifd
This also fixes the restricted/pure eval issues with callStackToNix.
I've also made the tests run in restricted mode to catch these kinds of
issues.