Fix typo.

Read md5sum passed to method directly.

slapos/recipe/build.py

@@ -162,7 +162,7 @@ class Script:
   def download(self, url, md5sum):
     download = zc.buildout.download.Download(self.buildout['buildout'],
         hash_name=True, cache=self.buildout['buildout'].get('download-cache'))
-    path, is_temp = download(url, md5sum=self.options.get('md5sum'))
+    path, is_temp = download(url, md5sum=md5sum)
     return path
 
   def extract(self, path):