jinja2: do nothing on update if we're sure that the source hasn't changed

See nexedi/slapos.recipe.build@f458eb8b

slapos/recipe/template/jinja2_template.py

@@ -274,5 +274,7 @@ class Recipe(object):
             return
         return self.rendered
 
-    update = install
+    def update(self):
+      if not self.md5sum:
+        self.install()