TEST-13-NSPAWN: explicitly create usr/lib/systemd directory in the container image

The main systemd package is not installed in the minimal image,
hence the directory may not exist.

test/units/TEST-13-NSPAWN.nspawn.sh

@@ -1586,6 +1586,7 @@ testcase_boot_param_split() {
     # Replace the init binary with a stub that records the argv and environment nspawn passes to it,
     # so we can verify that kernel-cmdline-style KEY=VALUE arguments are split between PID 1's
     # environment and argv the same way the kernel splits them.
+    mkdir -p "$root/usr/lib/systemd"
     cat >"$root/usr/lib/systemd/systemd" <<'EOF'
 #!/bin/bash
 set -e