duplicity: Disable pydrive2 support

It does not currently work due to pyopenssl incompatibility.

pkgs/by-name/du/duplicity/package.nix

···
         paramiko
         pyasn1
         pycrypto
-        pydrive2
+        # Currently marked as broken.
+        # pydrive2
         future
       ]
       ++ paramiko.optional-dependencies.invoke;