build-and-provide-package: use -b for package builds that shouldn't include sources

See dpkg-buildpackage(1):

| -b  Specifies a binary-only build, no source files are to be built and/or distributed. Passed to dpkg-genchanges.

 vs.

| -B  Specifies a binary-only build, limited to architecture dependent packages.  Passed to dpkg-genchanges.

We don't want to skip the architecture dependent packages but
just don't want to include sources in all the package builds
(since the sources can be included via reprepro in the repository
just *once*).

Thanks: alexander (user systemstart on github) for testing and feedback