Changes

debian-buster-jenkins: install ccache

Make ccache available inside debian-buster-jenkins. An upcoming osmo-ci
patch will make use of it in gerrit-verification and master jobs. With
this docker-playground patch alone it's not used yet since
/usr/lib/ccache is not in the default PATH.

Related: OS#5848
Change-Id: I164c7835f99436e8d768a864f36118eaf799d109