gnu: Improve memoization of 'package-with-bootstrap-guile'.

gnu: Improve memoization of 'package-with-bootstrap-guile'.

Before that, the command:

  GUIX_PROFILING=memoization guix build -e '(@@ (gnu packages commencement) gnu-make-final)' -nd

would show that 'package-with-bootstrap-guile' was called 2256
times (hit rate: 89%).  Now, it is called 745 times (hit rate: 85%).

"guix build libreoffice -nd" goes from 2.60s to 2.40s (-8%).

* gnu/packages/commencement.scm (gnu-make-final)
(coreutils-final, grep-final, sed-final, %final-inputs): Call
'package-with-bootstrap-guile' before 'package-with-explicit-inputs'.