introduces a 'boring' nixpkgs used by default for running VM tests
in this repo, i.e. do not force others to build my current version
of things like lix, which I overlay & which are part of any system
closure, but not actually important for testing.

this is, in the widest possible sense, a revert of
e88fed18f499a3e8ac98c772bbb62f00d1f8d1d7, which was now a little over
two years ago.

Of course, lots of things have changed since then:
 - this uses npins instead of niv, which is both simpler and still
   maintained
 - i haven't brought back the old deploy lib; I still use
   deploy-rs (with some modifications) to deploy things
 - if you actually use my stuff downstream, you can now use packages/ &
   tests/ & modules/ as entry points directly, while still having some
   control over inputs
 - (since i also don't believe any downstream users actually exist, i've
   not bothered to have a shim flake.nix so your stuff probably just
   broke. well, it was an experimental feature, anyways)
 - in general there's a lot more of the old-fashioned structure back
   again, with default.nix files in subdirectories that form a
   structure, not like how almost everything was just imported in the
   one big flake.nix file

For people who are interested in also having a non-flake config similar
to this one, it's probably best to take a look at inputs.nix (and also
at npins, of course)

unfortunately, these tests seem to be broken at the moment? Or at least
I cannot see where my packaging differs from the provided cpanfile.snapshot.

because like, why not learn how to use these out-of-tree?