(#15471) Ensure non-root can read report summary

The security fix for locking down the last_run_report, which contains
sensitive information, also locked down the last_run_summary, which does
not contain sensitive information. Unfortunately this file is often used
by monitoring systems so that they can track puppet runs. Since the
agent runs as root and the monitoring systems do not, this caused the
summary to become unreadable by the monitoring systems.

This commit returns the summary to being world readable which undoes
part of the change done in fd44bf5e6d0d360f6a493d663b653c121fa83c3f