autopkgtest code not updated on armhf remote workers

On 2022-09-19 there was an incident with the armhf remote workers being down and in the process of fixing it I discovered that the autopkgtest directory on those workers is never updated. While autopkgtest is called from the autopkgtest-lxd-worker the remote workers use autopkgtest-build-lxd to create (and update) the lxd containers which the armhf tests are run in. So if there is a fix in autopkgtest-build-lxd or an under lying bit of software we need to manually update the autopkgtest code on all the remote workers. Obviously this is less than ideal and we should either have the code automatically updated or a better process for updating the code on each unit.