buildd: on x86-grnet-03 install a sbuild wrapper

This wrapper add options to the sbuild command to:
- switch to unshare mode
- pass the name of chroot to use (in schroot mode, this is defined by
  files in /etc/schroot/chroot.d/)
- define the repositories to add to sources.list (in schroot mode, this
  is defined by 99builddsourceslist)

As debian-installer and debian-installer-netboot-images are known to
need network access, the wrapper fallback to schroot mode for them