Ubiquity crash while installing Jammy MATE

[Alkis Georgopoulos]

I was trying to install today's daily build of Ubuntu MATE 22.04, and a bit after the "user account data" page, ubiquity crashed.

UbiquitySyslog.txt shows that the problem is that /target/etc/apt/sources.list is missing, and a `grep` command fails the installation:

"""
Nov 20 08:35:18 ubuntu-mate ubiquity: grep: /target/etc/apt/sources.list
Nov 20 08:35:18 ubuntu-mate ubiquity: : No such file or directory
...
ubiquity.install_misc.InstallStepError: AptSetup failed with code 127
"""

ProblemType: Bug
DistroRelease: Ubuntu 22.04
Package: ubiquity 21.10.10
ProcVersionSignature: Ubuntu 5.13.0-19.19-generic 5.13.14
Uname: Linux 5.13.0-19-generic x86_64
NonfreeKernelModules: zfs zunicode zavl icp zcommon znvpair
ApportVersion: 2.20.11-0ubuntu73
Architecture: amd64
CasperMD5CheckResult: pass
CasperVersion: 1.465
Date: Sat Nov 20 10:36:15 2021
InstallCmdLine: BOOT_IMAGE=/casper/vmlinuz file=/cdrom/preseed/ubuntu-mate.seed maybe-ubiquity quiet splash ---
LiveMediaBuild: Ubuntu-MATE 22.04 LTS "Jammy Jellyfish" - Alpha amd64 (20211120)
SourcePackage: ubiquity
UpgradeStatus: No upgrade log present (probably fresh install)