I am trying to get Unattended-upgrades to work on my Linux Mint 18.3 (Sylvia) system, without success. My `etc/apt/apt.conf.d/50unattended-upgrades` is standard:

    Unattended-Upgrade::Allowed-Origins {
    	"${distro_id}:${distro_codename}";
    	"${distro_id}:${distro_codename}-security";
    etc.

but when I run it it never finds any upgrades. Here’s the output of `sudo unattended-upgrades -d`

    Initial blacklisted packages: 
    Initial whitelisted packages: 
    Starting unattended upgrades script
    Allowed origins are: ['o=LinuxMint,a=sylvia', 'o=LinuxMint,a=sylvia-security']
    Checking: unrar ([<Origin component:'multiverse' archive:'xenial-updates' origin:'Ubuntu' label:'Ubuntu' site:'archive.ubuntu.com' isTrusted:True>, <Origin component:'multiverse' archive:'xenial-security' origin:'Ubuntu' label:'Ubuntu' site:'security.ubuntu.com' isTrusted:True>])
    pkgs that look like they should be upgraded: 
    Fetched 0 B in 0s (0 B/s)                                                      
    fetch.run() result: 0
    blacklist: []
    whitelist: []
    No packages found that can be upgraded unattended and no pending auto-removals

But, I have an upgrade listed in the GUI Update Manager, for unrar-nonfree. The first line of the changelog is `unrar-nonfree (1:5.3.2-1+deb9u1build0.16.04.1) xenial-security; urgency=medium`

How can I make Unattended-upgrades look for xenial upgrades, as well as sylvia?