poudriere extract failures with TAR=/usr/local/bin/gtar with gtar from ports

[Graham Perrin]

The tail of a log from a failed run:

=======================<phase: extract        >============================
===>  License BSD3CLAUSE accepted by the user
===> Fetching all distfiles required by gflags-2.2.1 for building
===>  Extracting for gflags-2.2.1
=> SHA256 Checksum OK for gflags-gflags-v2.2.1_GH0.tar.gz.
/bin/sh: /usr/local/bin/gtar: not found
*** Error code 1

Stop.
make: stopped in /usr/ports/devel/gflags
=>> Cleaning up wrkdir
===>  Cleaning for gflags-2.2.1
build of devel/gflags | gflags-2.2.1 ended at Wed Oct 17 02:32:54 BST 2018
build time: 00:00:02
!!! build failure encountered !!!




----

Why not found? gtar _is_ present:

$ /usr/local/bin/gtar --version
tar (GNU tar) 1.30
Copyright (C) 2017 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <https://gnu.org/licenses/gpl.html>.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

Written by John Gilmore and Jay Fenlason.
$