No mailsync with locked GPG key in non-interactive shells

[ivan-boikov]

Assume you

• use pass with a password-protected GPG key
• clear cached passwords when locking screen
• run a local mail server
• use cron to run mailsync
• don't want to clear MAILTO in case actual errors happen

If so, cron will spam the server with "no secret key" emails. This one-liner prevents it.