Adding '/usr/bin/env bash' (#5334)

This commit aims to add '/usr/bin/env bash' as a shebang line
to indicates scripts use bash shell for interpreting.
Signed-off-by: Nguyen Hai Truong <truongnh@vn.fujitsu.com>

scripts/completions.bash

+#!/usr/bin/env bash
+
 # bash completion for helm                                 -*- shell-script -*-
 
 __debug()