service: Use explicit path for /usr/bin/env

A reddit thread[1] reported an error from env(1) invoked by service(8),
wwhen GNU coreutils utilities were installed without g prefixes.

[1] https://www.reddit.com/r/freebsd/comments/1kfzh78/service_command_changed_between_freebsd11_and/

Reviewed by: kevans
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D50195