feat(nix): Support package set imports from different sources

This extends the package set import mechanism in
build-registry-image.nix with several different options:

1. Importing a nixpkgs channel from Github (the default, pinned to
   nixos-19.03)

2. Importing a custom Nix git repository. This uses builtins.fetchGit
   and can thus rely on git/SSH configuration in the environment (such
   as keys)

3. Importing a local filesystem path

As long as the repository pointed at is either a checkout of nixpkgs,
or nixpkgs overlaid with custom packages this will work.

A special syntax has been defined for how these three options are
passed in, but users should not need to concern themselves with it as
it will be taken care of by the server component.

This relates to #3.

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