Add default.nix, shell.nix for compatibility

Dependencies

• [2] CLXEECMF * Start putting build results in a database.

Change contents

• file addition: default.nix (----------)
  [2.2]
  + (import (fetchTarball https://github.com/edolstra/flake-compat/archive/master.tar.gz) {

  + src = builtins.fetchGit ./.;

  + }).defaultNix
• file addition: shell.nix (----------)
  [2.2]
  + (import (fetchTarball https://github.com/edolstra/flake-compat/archive/master.tar.gz) {

  + src = builtins.fetchGit ./.;

  + }).shellNix