Note that the failure is due to stapbpf/Makefile.am trying to add the stapusr group, and the systemtap Debian package does that in systemtap-runtime's postinst script already.
Perhaps it would make sense to patch stapbpf/Makefile.am to get rid of install-exec-hook, and add a dpkg-statoverride call to systemtap-runtime.postinst for the stapbpf binary?
