mail/postfix: use ECHO_CMD instead of ECHO

• When using "make -s install" the build can fail because the essential modifications to the port build are not made correctly.
  1. make -s VECHO true
  2. make -s VECHO_CMD echo

PR: 222988
Submitted by: Franco Fichtner <franco@opnsense.org>
Approved by: ohauer (maintainer timeout)