please add shellcheck github action

[eslerm]

When I scanned the Ubuntu package openscap-1.3.7+dfsg-1 I found many shellcheck notes, warnings and errors. The errors I examined appeared serious and cause OpenSCAP to not work properly. notes do not seem significant, but some warnings are. They need their own triage and possibly assigned issues or CVEs. Here is the output of my local shellcheck scan: https://gist.github.com/eslerm/1430755c97570e46462d03c050e48c24

I would like to propose adding a shellcheck GitHub action to track these and new results.

[eslerm]

Please see https://github.com/OpenSCAP/openscap/pull/1929