[enhancement]: add automation for ubuntu/* braches to run shellcheck against changed package scripts (preinst, prerm, postrm, postinst)

[blackboxsw]

Enhancement

When debian/(pre|post)(rm|inst) scripts are modified in the current PR run shellcheck or some other tool to check that the number of POSIX shell warnings or errors is not increased by the active PR. This helps avoid POSIX shell logical errors or conditions from being introduced which could affect health of unattended-upgrades, do-release-upgrade or manual package updates by users.

Expect a non-blocking github action for this that is only run on pull requests targeting ubuntu/*