All of the existing tests only run on x86_64-linux and
the whole `nixpkgsFor` makes it hard to reuse those for
e.g. running the nixos tests with a sanitizer build of nix.
This rips off the bandaid and removes the `nixpkgsFor` parameter
in favor of a single instance of nixpkgs with a separate `nixComponents`.
... by moving our stuff out of the way from upstream's
`nixComponents` and `nixDependencies` attrsets.
(I prefer not to use overlays, but let's make it work this way
first)
