Add ruby style checker

[caguero]

Integrate rubocop as the ruby style checker.

@chapulina, @j-rivero, should we move the rubocop dependency from here to https://github.com/ignition-tooling/action-ignition-ci/blob/master/entrypoint.sh directly?

[chapulina]

Nice!

should we move the rubocop dependency from here to https://github.com/ignition-tooling/action-ignition-ci/blob/master/entrypoint.sh directly?

Since this is something that won't be needed by all packages, I'm leaning towards leaving it here.

[chapulina]

Since this is something that won't be needed by all packages,

Wait, come to think of it, most packages have a cmd<lib>.rb file that could be linted too 😏 And ign-math has more and more Ruby files too.

So now I think we could use some Ruby linting for most libraries. I guess the difficulty will be to put it in a common place to reduce duplication. Sort of how we have make codecheck on ign-cmake (which wouldn't help ign-tools...).

[chapulina]

This looks good to me as is, I'll leave it up to you if you think it could be moved to a common place.