wrapGAppsHook: Correct `wrapProgram` invocations

This change fixes several defects in the way `wrapGAppsHook` selected
the executable to wrap.

Previously, it would wrap any top-level files in the target `/bin` and
`/libexec` directories, including directories and non-executable
files.  In addition, it failed to wrap files in subdirectories.

Now, it uses `find` to iterate over these directory hierarchies,
selecting only executable files for wrapping.