build: force purging of sudo, ignore error message

Fixes bb758d3f, spotted by @Bubu:
DEBUG: buildserver > DEBUG: > sudo apt-get -y purge sudo
DEBUG: buildserver > Reading package lists...
DEBUG: buildserver > Building dependency tree...
DEBUG: buildserver > Reading state information...
DEBUG: buildserver > The following packages will be REMOVED:
DEBUG: buildserver >   sudo*
DEBUG: buildserver > 0 upgraded, 0 newly installed, 1 to remove and 0 not upgraded.
DEBUG: buildserver > After this operation, 2,391 kB disk space will be freed.
(Reading database ... 68491 files and directories currently installed.)
DEBUG: buildserver > Removing sudo (1.8.10p3-1+deb8u4) ...
DEBUG: buildserver > You have asked that the sudo package be removed,
DEBUG: buildserver > but no root password has been set.
DEBUG: buildserver > Without sudo, you may not be able to gain administrative privileges.
DEBUG: buildserver >
DEBUG: buildserver > If you would prefer to access the root account with su(1)
DEBUG: buildserver > or by logging in directly,
DEBUG: buildserver > you must set a root password with "sudo passwd".
DEBUG: buildserver >
DEBUG: buildserver > If you have arranged other means to access the root account,
DEBUG: buildserver > and you are sure this is what you want,
DEBUG: buildserver > you may bypass this check by setting an environment variable
DEBUG: buildserver > (export SUDO_FORCE_REMOVE=yes).
DEBUG: buildserver >
DEBUG: buildserver > Refusing to remove sudo.
DEBUG: buildserver > dpkg: error processing package sudo (--purge):
DEBUG: buildserver >  subprocess installed pre-removal script returned error exit status 1
DEBUG: buildserver > Errors were encountered while processing:
DEBUG: buildserver >  sudo
DEBUG: buildserver > E: Sub-process /usr/bin/dpkg returned an error code (1)
2 jobs for security-fixes in 44 minutes and 33 seconds (queued for 2 minutes and 52 seconds)
Status Job ID Name Coverage
  Test
passed #44607715
metadata_v0

00:04:17

passed #44607714
test

00:44:33