Use bash in systemd-entrypoint shebang (#4008)

Uses a shell environment where `-o pipefail` is available.

Closes #4005

Signed-off-by: Cole White <cwhite@wikimedia.org>

distribution/packages/src/common/systemd/systemd-entrypoint

@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/usr/bin/env bash
 
 # This wrapper script allows SystemD to feed a file containing a passphrase into
 # the main OpenSearch startup script