python3Packages.polars: fix the mimalloc flag

Since https://github.com/pola-rs/polars/pull/16365, the proper way to
use the mimalloc allocator is the allocator=mimalloc config options,
instead of use_mimalloc.

Update our packaging accordingly

pkgs/development/python-modules/polars/default.nix

@@ -97,7 +97,7 @@ buildPythonPackage rec {
 
     RUSTFLAGS = lib.concatStringsSep " " (
       lib.optionals (polarsMemoryAllocator.pname == "mimalloc") [
-        "--cfg use_mimalloc"
+        "--cfg allocator=\"mimalloc\""
       ]
     );
     RUST_BACKTRACE = true;