Use ai3/tools/smtpd-pipe instead of the builtin smtpd.go

The smtpd-pipe tool has been built explicitly to take over this
little binary.

Fixes issue #2.
2 jobs for s6 in 37 seconds (queued for 7 seconds)
Status Name Job ID Coverage
  Build
passed build #42700

00:00:22

passed trivy #42701

00:00:15