python3Packages.pystemmer: fix build with structuredAttrs (#470711)

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

@@ -40,9 +40,10 @@ buildPythonPackage rec {
     export PYSTEMMER_SYSTEM_LIBSTEMMER="${lib.getDev libstemmer}/include"
   '';
 
-  env.NIX_CFLAGS_COMPILE = toString [ "-I${lib.getDev libstemmer}/include" ];
-
-  NIX_CFLAGS_LINK = [ "-L${libstemmer}/lib" ];
+  env = {
+    NIX_CFLAGS_COMPILE = toString [ "-I${lib.getDev libstemmer}/include" ];
+    NIX_CFLAGS_LINK = toString [ "-L${libstemmer}/lib" ];
+  };
 
   pythonImportsCheck = [ "Stemmer" ];
 
@@ -52,6 +53,8 @@ buildPythonPackage rec {
     runHook postCheck
   '';
 
+  __structuredAttrs = true;
+
   meta = {
     description = "Snowball stemming algorithms, for information retrieval";
     downloadPage = "https://github.com/snowballstem/pystemmer";