only run CI GA when there is a change in common.vars.json

[dragarcia]

A change in common.vars.json would signify a bump in the version. Right now, any change in the develop branch even from just the README would trigger a build of the AMI. This would cause the GA to immediately fail as the image name already exists.

[darora]

How about instead we start using the commit convention we use in the rest of our repos, and version off that?

Only building on common.vars.json has the downside of not building again if e.g. you're fixing a broken build (all too common).