mkosi: use the just built systemd-sysusers instead of groupadd

It disappeared again from SUSE Tumbleweed. Just use sysusers since we
just built it.

mkosi.build

@@ -156,7 +156,7 @@ cd "$BUILDDIR"
 ninja "$@"
 if [ "$WITH_TESTS" = 1 ] ; then
         for id in 1 2 3; do
-                getent group $id >/dev/null || groupadd -g $id testgroup$id
+                getent group $id >/dev/null || echo "g testgroup$id $id -" | ./systemd-sysusers -
         done
 
         if [ -n "$SANITIZERS" ]; then