postfixadmin: downgrade meta.platforms to *unix except darwin*

Because the package uses `/etc` and `/var/cache`

pkgs/by-name/po/postfixadmin/package.nix

@@ -40,6 +40,6 @@ stdenv.mkDerivation (finalAttrs: {
     homepage = "https://postfixadmin.sourceforge.io/";
     maintainers = with maintainers; [ globin ];
     license = licenses.gpl2Plus;
-    platforms = platforms.all;
+    platforms = lib.subtractLists platforms.darwin platforms.unix; # There is no /var/cache/ on MacOS
   };
 })