Reading user crontab on unix as that user

Crontab on Solaris and Illumos systems with auditing enabled
audits root attempts to read non-root user crontabs. If chef-client
runs without an auditing context (as happens by default when running
in SMF), these crontab calls will fails with exit status 1.

By switching to the non-root user first (which somehow does not
require auditing), we should be able to read user crontabs without
an auditing context.