postgresqlPackages.{plperl,plpython3,plr}.withPackages: fix

This broke in #533324 - `postBuild` is a regular argument for
`buildEnv`, not part of `derivationArgs`.

pkgs/servers/sql/postgresql/wrapper.nix

@@ -40,15 +40,16 @@ buildEnv (finalAttrs: {
   derivationArgs = {
     strictDeps = true;
     nativeBuildInputs = [ makeBinaryWrapper ];
-    postBuild =
-      let
-        args = lib.concatMap (ext: ext.wrapperArgs or [ ]) installedExtensions;
-      in
-      ''
-        wrapProgram "$out/bin/postgres" ${lib.concatStringsSep " " args}
-      '';
   };
 
+  postBuild =
+    let
+      args = lib.concatMap (ext: ext.wrapperArgs or [ ]) installedExtensions;
+    in
+    ''
+      wrapProgram "$out/bin/postgres" ${lib.concatStringsSep " " args}
+    '';
+
   passthru = {
     inherit installedExtensions;
     inherit (postgresql)