buildRustPackage: warn on explicit useFetchCargoVendor

pkgs/build-support/rust/build-rust-package/default.nix

···
     }@args:
 
     assert lib.assertMsg useFetchCargoVendor
-      "buildRustPackage: `useFetchCargoVendor` is non‐optional and enabled by default as of 25.05";
+      "buildRustPackage: `useFetchCargoVendor` is non‐optional and enabled by default as of 25.05, remove it";
+
+    assert lib.warnIf (args ? useFetchCargoVendor)
+      "buildRustPackage: `useFetchCargoVendor` is non‐optional and enabled by default as of 25.05, remove it"
+      true;
 
     lib.optionalAttrs (stdenv.hostPlatform.isDarwin && buildType == "debug") {
       RUSTFLAGS = "-C split-debuginfo=packed " + (args.RUSTFLAGS or "");