packages: Make create-sets.sh more robust

Use ${PKG_CMD} rather than bare 'pkg' to fix the build when pkg is
not in the tools path.  Provide a default in case it's not set for
some reason (e.g., running the script by hand).

Since set -- $(...) does not trigger an exit from set -e if the
command fails, this failure was silent and resulted in sets not
being built correctly if we failed to run pkg.  Use a temporary
variable, which does trigger set -e, to fail correctly.

MFC after:	2 weeks
Reviewed by:	sjg
Sponsored by:	https://www.patreon.com/bsdivy
Differential Revision:	https://reviews.freebsd.org/D56676

release/packages/create-sets.sh

@@ -26,6 +26,8 @@ if [ $# -lt 3 ]; then
 	exit 1
 fi
 
+: ${PKG_CMD:="pkg"}
+
 srcdir="$1"; shift
 wstagedir="$1"; shift
 repodir="$1"; shift
@@ -43,7 +45,8 @@ for pkg in "$repodir"/*.pkg; do
 	# packages.
 	{ echo "$pkg" | grep -q '-'; } || continue
 
-	set -- $(pkg query -F "$pkg" '%At %n %Av' | grep '^set ')
+	_tmp="$(${PKG_CMD} query -F "$pkg" '%At %n %Av' | grep '^set ')"
+	set -- $_tmp
 	pkgname="$2"
 	sets="$(echo "$3" | tr , ' ')"
 	for set in $sets; do