Only run the "Verify images" part of the lint workflow if any images were changed

[cooljeanius]

Right now the lint workflow takes awhile due to the large number of images that woptipng needs to check; I think GitHub Actions supports a paths key or something that we could set so that it only runs when the mentioned paths are changed...

[cooljeanius]

I think GitHub Actions supports a paths key or something that we could set so that it only runs when the mentioned paths are changed...

...yeah, the map-diff workflow already uses it so that it only runs if any maps are changed...

[cooljeanius]

...of course, part of the issue here is just that there are so many images to check in the first place, which is something that fixing issue #23 ought to help with...

[cooljeanius]

See also czyzby/wesnoth-png-optimizer#7