Add shellcheck to Github Actions

[lukaszstolarczuk]

While the change introduced in the PR: #749 was nice, it wasn't completely done, and now (after rebasing to the latest master), it produces many errors. I'm not sure if all the checks for bash scripts are required, perhaps some of them are "redundant" or just "too aggressive"... or perhaps we should just fix them one by one. For now, it is not our highest priority.

Besides, I'm not sure if tests on our CI were executed properly - my guess is the change around test.sh (which -> command -v to be precise) may lead to unexpected behavior. It would require some deeper diving into this problem.