Running `make install_depends` does not (always) pull python3-systemd
which is a dependency of Subiquity.
In environments having Install-Recommends enabled, python3-systemd is
pulled by apport which is itself pulled by python3-apport.
Fixed by adding it to the list of dependencies in apt-deps.txt
Signed-off-by: Olivier Gayot <olivier.gayot@canonical.com>