[canvaskit] Make local-example explicitly use Python2

The script doesn't work with Python3.

Change-Id: I9ea6397506500f76570540f78aa736873769f92b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/403016
Reviewed-by: Kevin Lubick <kjlubick@google.com>

modules/canvaskit/Makefile

@@ -98,7 +98,7 @@ local-example:
 	mkdir -p node_modules
 	ln -s ../npm_build node_modules/canvaskit
 	echo "Go check out http://localhost:8000/npm_build/example.html"
-	python serve.py
+	python2 serve.py
 
 test-continuous:
 	echo "Assuming npm ci has been run by user"