stream through proxy when using fileRetriever

The problem was that when the proxy requests a key be retrieved to its
own temp file, fileRetriever was retriving it to the key's temp
location, and then moving it at the end, which broke streaming.

So, plumb through the path where the key is being retrieved to.

Remote/Directory.hs

@@ -173,7 +173,7 @@ locations d k = NE.map (d P.</>) (keyPaths k)
 locations' :: RawFilePath -> Key -> [RawFilePath]
 locations' d k = NE.toList (locations d k)
 
-{- Returns the location off a Key in the directory. If the key is
+{- Returns the location of a Key in the directory. If the key is
  - present, returns the location that is actually used, otherwise
  - returns the first, default location. -}
 getLocation :: RawFilePath -> Key -> IO RawFilePath