This is to remove code duplication in "emacs/guix-main.scm" and
"guix/scripts/package.scm" ('build-and-use-profile' did not exist until
commits 1b67644 and d1ac5c0).

The only important (or maybe not) difference is:

  (set-guile-for-build (default-guile))

in "guix-main.scm" which does not exist in 'build-and-use-profile'.  It
was introduced in commit 4ad2e76 and I don't really understand why it is
needed.