deltachat-desktop: fix build of libdeltachat

pkgs/applications/networking/instant-messengers/deltachat-desktop/default.nix

···
     src = fetchFromGitHub {
       owner = "deltachat";
       repo = "deltachat-core-rust";
-      rev = version;
+      rev = "v${version}";
       hash = "sha256-bvXZtgFZx94Sw9Tst620HAhi9kmG8PjtWnghdw2ZF84=";
     };
     cargoDeps = rustPlatform.importCargoLock {