python: fix modules in buildEnv, closes #16646

Python 2.7 on Nix is stripped of some modules, which can be added when
needed. This was broken with python.buildEnv.

pkgs/development/interpreters/python/2.7/default.nix

···
 
       buildInputs = [ python ] ++ deps;
 
+      # We need to set this for python.buildEnv
+      pythonPath = [];
+
       inherit (mkPaths buildInputs) C_INCLUDE_PATH LIBRARY_PATH;
 
       # non-python gdbm has a libintl dependency on i686-cygwin, not on x86_64-cygwin