With a manifest file, all the packages necessary for the ELPA
build-system to work are provided within the new environment.  All that
has to be changed (see last patch) is to tell bubblewrap that the /gnu
directory should be visible, as "guix shell" adds a profile from within
that directory to the PATH.

I also noticed that there are issues when running "make -B" because
mkdir doesn't want to re-create the "packages" directory.  So this
change should be non-controversial: