Looks like the host unattended upgrades is interferring with chroot
creation.  Here, we upgrade machine image to one without unattended
upgrades misconfigured, across the board except for the `test-docker`
job.

Docker seems to be misbehaving on that image, and we get some unexpected
cgroups errors, e.g. https://circleci.com/gh/hashicorp/nomad/3854 .

Sample recent failures of `test-exec`:

https://circleci.com/gh/hashicorp/nomad/3633
https://circleci.com/gh/hashicorp/nomad/3696
https://circleci.com/gh/hashicorp/nomad/3714
https://circleci.com/gh/hashicorp/nomad/3764
https://circleci.com/gh/hashicorp/nomad/3770
https://circleci.com/gh/hashicorp/nomad/3834