gnu: bootstrap: 'bootstrap-origin' preserves eq?-ness when no changes are made.

This reduces the number of lookups in the 'add-data-to-store' cache from
2743 to 2705 (hit rate: 11% to 10%) when running

  GUIX_PROFILING=add-data-to-store-cache guix build libreoffice -nd

The execution time of "guix build libreoffice -nd" goes from 1.80s to 1.78s.

* gnu/packages/bootstrap.scm (bootstrap-origin): Return SOURCE unchanged
when its has no patches and no snippet.