Code review comment for ~paelzer/ubuntu/+source/pollinate:lp-1848923-cache-directory-HIRSUTE

lgtm. However, note that the CacheDirectory directive doesn't appear available in Xenial (at least I don't see it in http://manpages.ubuntu.com/manpages/xenial/en/man7/systemd.directives.7.html but it is in http://manpages.ubuntu.com/manpages/bionic/en/man7/systemd.directives.7.html). The pollinate has been backported from Focal to Xenial before. A decision needs to be made on whether we want to retain backwards compatibility back to Xenial on the packaging in Hirsute, or suffer more backporting pain instead. If the latter, we need to ensure that this doesn't get missed if the package is backported.