MDEV-21869: Make Debian packaging Lintian clean

Fix Lintian complaints on:
 P: maintainer-script-without-set-e
 W: mariadb-server-10.5: script-not-executable usr/share/mysql/mysql.init
 W: mariadb-client-10.5: incorrect-path-for-interpreter usr/bin/innotop
    (#!/usr/bin/env perl != /usr/bin/perl)

debian/additions/innotop/innotop

@@ -1,4 +1,4 @@
-#!/usr/bin/env perl
+#!/usr/bin/perl
 
 # vim: tw=160:nowrap:expandtab:tabstop=3:shiftwidth=3:softtabstop=3
 

debian/mariadb-common.postrm

@@ -1,4 +1,5 @@
-#!/bin/bash -e
+#!/bin/bash
+set -e
 
 if [ "$1" = "purge" ]; then
   rmdir /etc/mysql/conf.d 2>/dev/null || true

debian/mysql-common.postrm

@@ -1,4 +1,5 @@
-#!/bin/bash -e
+#!/bin/bash
+set -e
 
 if [ "$1" = "purge" ]; then
   rmdir /etc/mysql 2>/dev/null || true