guix build: Improve procedural decomposition.

* guix/scripts/build.scm (%store): Remove.
  (derivation-from-expression): Add 'store' parameter.  Adjust caller
  accordingly.
  (register-root): New procedure, formerly within 'guix-build'.
  (options->derivations): New procedure, formerly inline within
  'guix-build'.
  (guix-build): Adjust accordingly.