Have cl65 run the binaries from its own directory.

While ca65, cc65 and ld65 have built-in paths cl65 doesn't.
That means that up to now cl65 depended on the binaries
being found in the path env var. However it makes sense
to presume that the binaries are located in the very same
directory cl65 is located in. So whatever pathname was
suitable to run cl65 should be suitable to run the other
binaries too. But if for some reason there's no valid
argv[0] or if it doesn't contain at least one directory
delimiter ('/' or '\') then fall back to relying on the
path env var.