Let it use debian bullseye and rename the directory once, so we don't
need to rename it in the future when switching the debian version
again.
The reason I'm doing this now is that building with debian 9 failed
due to obs.osmocom.org related changes. It would be trivial to fix that,
but I decided to rather upgrade to the latest debian version.
Change-Id: Ib6039724a7200abce5252f95407047296b72cd51
Oliver Smith