scion: replace rev with tag

pkgs/by-name/sc/scion/package.nix

···
   src = fetchFromGitHub {
     owner = "scionproto";
     repo = "scion";
-    rev = "v${finalAttrs.version}";
+    tag = "v${finalAttrs.version}";
     hash = "sha256-J51GIQQhS623wFUU5dI/TwT2rkDH69518lpdCLZ/iM0=";
   };