release: Use PKG_CMD when installing pkgbase packages in vmimage.subr

This defaults to plain "pkg", but being able to override it is useful
when testing pkg itself.

Reviewed by:	cperciva
MFC after:	3 days
Differential Revision:	https://reviews.freebsd.org/D53307

(cherry picked from commit bbe65c5e8c0e89ec14cb5d00153691850dadb859)

release/tools/vmimage.subr

@@ -97,7 +97,7 @@ vm_install_base() {
 
 	if [ -z "${NOPKGBASE}" ]; then
 		local pkg_cmd
-		pkg_cmd="pkg --rootdir ${DESTDIR} --repo-conf-dir ${PKGBASE_REPO_DIR}
+		pkg_cmd="${PKG_CMD} --rootdir ${DESTDIR} --repo-conf-dir ${PKGBASE_REPO_DIR}
 			-o ASSUME_ALWAYS_YES=yes -o IGNORE_OSVERSION=yes
 			-o ABI=${PKG_ABI} -o INSTALL_AS_USER=yes "
 		if [ -n "${NO_ROOT}" ]; then