[Leapp] `dnf config-manager setopt` to exclude packages fails with a python traceback

Issue

Leapp logs report the following python traceback (truncated):

CalledProcessError: Command ['systemd-nspawn', '--register=no', '--quiet', '-D', u'/var/lib/leapp/el8userspace', '--setenv=LEAPP_UPGRADE_PATH_TARGET_RELEASE=8.10', '--setenv=LEAPP_NO_RHSM=0', '--setenv=LEAPP_EXPERIMENTAL=0', '--setenv=LEAPP_TARGET_PRODUCT_CHANNEL=ga', '--setenv=LEAPP_UPGRADE_PATH_FLAVOUR=default', '--setenv=LEAPP_COMMON_TOOLS=:/etc/leapp/repos.d/system_upgrade/common/tools:/etc/leapp/repos.d/system_upgrade/el7toel8/tools', '--setenv=LEAPP_COMMON_FILES=:/etc/leapp/repos.d/system_upgrade/common/files:/etc/leapp/repos.d/system_upgrade/el7toel8/files', '--setenv=LEAPP_IPU_IN_PROGRESS=7to8', '--setenv=LEAPP_UNSUPPORTED=0', '--setenv=LEAPP_EXECUTION_ID=bd2c4280-bc1d-4e73-b41f-9cd64c4ba82b', '--setenv=LEAPP_HOSTNAME=localhost.localdomain', 'dnf', 'config-manager', '--save', '--setopt', 'exclude=python2-leapp,snactor,leapp-upgrade-el7toel8,leapp'] failed with exit code 1.