This PR allows to call the appimage-builder action with multiple arguments.

When an action is defined with using: docker, args must be an array. This means we have to create a JSON list from appimage-builder, --recipe=${{inputs.recipe}} and inputs.args. As far as I know, this is not possible within a single docker action because it cannot be done with expressions only.

When an action is defined with using: composite, args is a single string. That means we can simply concat appimage-builder, --recipe=${{inputs.recipe}} and inputs.args.

Fixes #7