Do not raise an error if os.environ does not contain PATH

[radarhere]

Resolves #6934

Pillow/setup.py

Lines 245 to 249 in 7e3bed6

	def _cmd_exists(cmd):
	return any(
	os.access(os.path.join(path, cmd), os.X_OK)
	for path in os.environ["PATH"].split(os.pathsep)
	)

presumes that os.environ contains a "PATH" key.

This PR changes the code no longer raise an error if it doesn't.