This reduces the number of lookups in the 'add-data-to-store' cache from
2705 to 2685 (hit rate: 10% to 9%) when running:
GUIX_PROFILING=add-data-to-store-cache guix build libreoffice -nd
* gnu/packages/bootstrap.scm (raw-build)[->store]: Use 'lower-object'
instead of 'origin->derivation'. This allows the origin-to-derivation
mapping to be cached.