enhance the installation detection of involucro

[bgruening]

fixes: #110

[mbargull]

distutils.spawn.find_executable only does half the job of shutil.which, i.e., it is only checking for existence but not user executability.
I think (given a shutil.which implementation) you'd want something like

if not os.path.dirname(self.involucro_bin):
    return shutil.which(self.involucro_bin)
return (os.access(self.involucro_bin, os.F_OK | os.X_OK) and not os.path.isdir(self.involucro_bin))

[bgruening]

@mbargull missed that, sorry. Do you think if we find the binary, but it is not executable, that we should install/download a new binary? We should probably error out, isn't it?