unattended-upgrades runs with duplicate sources

Since focal (as far as LTS releases are concerned), unattended-upgrades ships 50unattended-upgrades which ends up installed in /etc/apt/apt.conf.d/50unattended-upgrades

Before running unattended-upgrades, subiquity creates the /etc/apt/apt.conf.d/zzzz-temp-installer-unattended-upgrade file (with or without -updates enabled).

But unattended-upgrade reads the directives from both files, and we end up with duplicate sources:

2023-04-17 08:39:48,779 INFO Allowed origins are: o=Ubuntu,a=lunar, o=Ubuntu,a=lunar-security, o=UbuntuESMApps,a=lunar-apps-security, o=UbuntuESM,a=lunar-infra-security, o=Ubuntu,a=lunar, o=Ubuntu,a=lunar-security, o=UbuntuESMApps,a=lunar-apps-security, o=UbuntuESM,a=lunar-infra-security

Furthermore, if 50unattended-upgrades started enabling -updates, it would defeat subiquity's ability to pull only -security.