Previously, the Ansible provisioner would look for the username from the
`USER` environment variable. Unfortunately, this is not always set -
particularly in Docker containers. It's very confusing to understand why
the error is happening.
Switched to using Go's built-in `os/user` package for retrieving the
current username. @rickard-von-essen had done this in 7369841, but
moved away from it in d59844f because, at the time, it wasn't possible
to use that library with cross-compilation. This was fixed in Go in
795e712b72
b894c925d1