acme: remove unneeded 'nose' dep, use 'sourceRoot' instead of postUnpack

Specifying "source" in sourceRoot isn't ideal
(hopefully it doesn't change, again?)
but I think this is the preferred way to do this.

pkgs/development/python-modules/acme/default.nix

@@ -1,6 +1,5 @@
 { buildPythonPackage
 , certbot
-, nose
 , pytest
 , cryptography
 , pyasn1
@@ -26,9 +25,9 @@ buildPythonPackage rec {
     werkzeug mock ndg-httpsclient josepy
   ];
 
-  checkInputs = [ nose pytest ];
+  checkInputs = [ pytest ];
 
-  postUnpack = "sourceRoot=\${sourceRoot}/acme";
+  sourceRoot = "source/${pname}";
 
   meta = certbot.meta // {
     description = "ACME protocol implementation in Python";