CHROOT_SESSION_PURGE is set to false without union-type, so 90apt-sources does not render

I'm not sure if this is incorrect behavior in schroot or incorrect assumption in sbuild-launchpad-chroot.

I have a system where I was not able to use union, so the focal-amd64 config that was built by sbuild-launchpad-chroot looks like below. When attempting to build from it (sbuild --dist=focal --arch=amd64 --arch-all package.dsc), CHROOT_SESSION_PURGE is set to false. That causes me to take the 'exit 0' path in https://git.launchpad.net/~motu/ubuntu/+source/sbuild-launchpad-chroot/tree/etc/schroot/setup.d/90apt-sources#n39 .

The end result is the 'apt.default.mirror' setting is not honored.

If you set 'union-type=overlay', then you'll get CHROOT_SESSION_PURGE set to true and it will work as desired.

% cat /etc/schroot/chroot.d/focal-amd64

[focal-amd64]
type=directory
directory=/var/lib/schroot/chroot/focal-amd64
description=Ubuntu focal/amd64 sbuild
root-groups=root,sbuild
profile=sbuild
launchpad.dist=ubuntu
launchpad.series=focal
launchpad.arch=amd64
launchpad.url=http://launchpadlibrarian.net/475801990/livecd.ubuntu-base.rootfs.tar.gz
apt.enable=true
aliases=focal-security-amd64,focal-security+main-amd64,focal-security+restricted-amd64,focal-security+universe-amd64,focal-security+multiverse-amd64,focal-updates-amd64,focal-updates+main-amd64,focal-updates+restricted-amd64,focal-updates+universe-amd64,focal-updates+multiverse-amd64,focal-proposed-amd64,focal-proposed+main-amd64,focal-proposed+restricted-amd64,focal-proposed+universe-amd64,focal-proposed+multiverse-amd64,focal-backports-amd64,focal-backports+main-amd64,focal-backports+restricted-amd64,focal-backports+universe-amd64,focal-backports+multiverse-amd64
apt.default.mirror=MY_MIRROR

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: sbuild-launchpad-chroot 0.17ubuntu0.20.10.1~20.04.1
ProcVersionSignature: Ubuntu 5.13.0-28.31~20.04.1-generic 5.13.19
Uname: Linux 5.13.0-28-generic x86_64
NonfreeKernelModules: zfs zunicode zavl icp zcommon znvpair
ApportVersion: 2.20.11-0ubuntu27.21
Architecture: amd64
CasperMD5CheckResult: skip
CurrentDesktop: ubuntu:GNOME
Date: Tue Apr 12 15:53:54 2022
InstallationDate: Installed on 2020-01-15 (817 days ago)
InstallationMedia: Ubuntu 18.04.3 LTS "Bionic Beaver" - Release amd64 (20190805)
PackageArchitecture: all
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: sbuild-launchpad-chroot
UpgradeStatus: Upgraded to focal on 2020-04-17 (725 days ago)
modified.conffile..etc.schroot.setup.d.90apt-sources: [modified]
mtime.conffile..etc.schroot.setup.d.90apt-sources: 2022-04-12T15:23:14.405987