dump-images: Execute glretrace from source dir when running uninstalled

Previously, we've had code to carefully find wrappers and scripts
relative to an apitrace binary being run from an uninstalled
directory. This is extremely useful while testing an experimental
feature before installing said experimental code.

Similarly, provide a findProgram function to do the same thing for an
executable program and use it within "apitrace dump-images" when
invoking glretrace.