System-tests: Use bash explicitly
On Ubuntu, /bin/sh != /bin/bash.  Update system-tests to only use
bash for testing consistency across platforms.
@@ -22,7 +22,7 @@ load helpers
# as a user, the parent directory must be world-readable.
test_script=$PODMAN_TMPDIR/fail-if-writable
cat >$test_script <<"EOF"
-#!/bin/sh
+#!/bin/bash
path="$1"
