mwscript-mwcron: Make logging optional
Followup to 5e1c07a9. The added logging is important for periodic jobs, but with mwscript-k8s also using this as an entry point, we don't want the added output in the interactive (or shell pipeline) environment. This allows us to switch it off by setting MWSCRIPT_MWCRON_QUIET=1 when launching.