Use same shell across all Debian packaging scriptlets

[legoktm]

copied from freedomofpress/securedrop-client#1878

Description

We currently inconsistently use bash and dash (/bin/sh) when executing postinst/prerm/etc. scripts in Debian packaging:

user@dev ~/g/f/securedrop (develop)> rg bin/sh securedrop/debian/
securedrop/debian/securedrop-ossec-agent.postinst
1:#!/bin/sh

securedrop/debian/securedrop-app-code.postrm
1:#!/bin/sh

securedrop/debian/securedrop-ossec-server.postinst
1:#!/bin/sh

securedrop/debian/securedrop-config.postinst
1:#!/bin/sh

securedrop/debian/app-code/etc/apparmor.d/usr.sbin.apache2
103:  /usr/bin/shred rix,
user@dev ~/g/f/securedrop (develop)> rg bin/bash securedrop/debian/
securedrop/debian/securedrop-app-code.preinst
1:#!/bin/bash

securedrop/debian/securedrop-app-code.postinst
1:#!/bin/bash

securedrop/debian/securedrop-app-code.config
1:#!/bin/bash

In the case of securedrop-export.postrm, I had used bash syntax in freedomofpress/securedrop-client@a56b866 except it's shebang is plain /bin/sh, so it uses the POSIX dash shell, causing issues.

My preference is using bash everywhere unconditionally, using dash gives a small performance benefit but given the number of packages we're installing, it's pretty minor IMO (unlike across all of Debian) versus the nicer feature set of bash.