bug#22459: Guix tools should not honor NIX_* environment variables

[Ricardo Wurmus]

These NIX_* variables are still in use:

  NIX_AFFINITY_HACK
  NIX_BIN_DIR
  NIX_BUILD_CORES
  NIX_HELD_LOCKS
  NIX_IGNORE_SYMLINK_STORE
  NIX_STORE
  NIX_STORE_DIR

This is used internally:

  _NIX_OPTIONS

-- 
Ricardo