Nix Overrides That Expire Themselves

A practical Nix pattern for writing package overrides that warn you when they've become obsolete. Using `lib.warnIf` and `lib.versionAtLeast`, an override compares the pinned nixpkgs version against the required version and prints a warning during evaluation when nixpkgs has caught up. The same technique applies to overrides that mark packages as broken — `lib.warnIfNot` fires when the broken flag is eventually removed upstream, prompting cleanup of the workaround.