cargo-deadlinks: fix darwin and aarch64-linux test failures

pkgs/development/tools/rust/cargo-deadlinks/default.nix

···
   checkFlags = [
     # uses internet
     "--skip non_existent_http_link --skip working_http_check"
-  ];
+  ] ++ lib.optional (stdenv.hostPlatform.system != "x86_64-linux")
+    # assumes the target is x86_64-unknown-linux-gnu
+    "--skip simple_project::it_checks_okay_project_correctly";
 
   buildInputs = lib.optional stdenv.isDarwin Security;