python3Packages.proton-client: change platform to linux

pkgs/development/python-modules/proton-client/default.nix

···
     homepage = "https://github.com/ProtonMail/proton-python-client";
     license = licenses.gpl3Only;
     maintainers = with maintainers; [ wolfangaukang ];
+    platforms = platforms.linux;
   };
 }