images: updating the gimp_exe binary to a wrapping script.

In order to fix local builds for macOS, I am wrapping our locally built executable in a shell script which will additionally update the rpath on macOS. See gimp!1457 (merged).

A consequence of this is that we need to add a dependency to the built executable (gimp_exe used to be it, but now it's just a script in the source repository, so we add a gimp_exe_depends).