fix source root locations.

These have changed following commit
c3255fe8ec.

See also:

- https://github.com/NixOS/nixpkgs/pull/31004
- https://github.com/NixOS/nixpkgs/pull/31106

pkgs/development/libraries/cxxtest/default.nix

@@ -13,7 +13,9 @@ in python2Packages.buildPythonApplication rec {
     sha256 = "19w92kipfhp5wvs47l0qpibn3x49sbmvkk91yxw6nwk6fafcdl17";
   };
 
-  sourceRoot = "${name}-src/python";
+  setSourceRoot = ''
+    sourceRoot=$(echo */python)
+  '';
 
   meta = with stdenv.lib; {
     homepage = http://cxxtest.com;