s390x-se-luks-gencpio: Use interoperable path for bash

On OSs that do not consistently merge /usr/bin with /bin, the path to
bash has traditionally been /bin/bash.

Signed-off-by: Simon McVittie <smcv@debian.org>

src/libostree/s390x-se-luks-gencpio

@@ -1,4 +1,4 @@
-#!/usr/bin/bash
+#!/bin/bash
 # This script creates new initramdisk with LUKS config within
 set -euo pipefail