derivations: Don't memoize 'derivation->bytevector'.

Its hit rate was only 8%.  Removing it reduces heap size of "guix build
libreoffice -nd" from 69MiB to 61MiB and the wall-clock time is unchanged.

* guix/derivations.scm (derivation->bytevector): Change from 'mlambda'
to 'lambda'.