[nix-local-build] new-install interface for installing executables

[ezyang]

Suppose you want to install pandoc. How would you do so using the Nix-local build interface? new-install is not supported, so you can't. Installing executables is, indeed, a really good reason why we need new-install: to manage a "Nix profile", which is the actual set of packages available to a user's PATH. I suppose we also want the ability to rollback (like in Nix) in case someone installs something they don't like. These would also be considered roots for any sort of hypothetical GC mechanism.

Here's an even easier way to do it: to new-install an executable, first install it into the store, and then create a symlink from .cabal/bin to this executable.

(@hvr says: see also #4558)