This commit adjusts #232576 to break the
fetchurl<->minimal-bootstrap-sources dependency cycle without
needing an upload to tarballs.nixos.org.  It does this by appending
the low-level FOD attributes onto the `runCommand` derivation.

  https://nixos.org/manual/nix/unstable/language/advanced-attributes.html#adv-attr-outputHash

Ericson2314 observed that although FODs break the build-time cycle,
they don't break the eval-time cycle.  This cycle must be broken in
order to compute the derivation hash.

make-minimal-bootstrap-sources does and always has depended on the
full nixpkgs (including fetchFromGitHub).  This commit completely
separates minimal-bootstrap-sources from it, so that
minimal-bootstrap-sources now does not depend on
make-minimal-bootstrap-sources (or nixpkgs) in any way.

minimal-bootstrap-sources is now a "bare Nix" derivation.  It is an
FOD whose builder always fails with an error message.