Replace recursive `final.lib.optionalAttrs` with `prev.lib.optionalAttrs`
using the `?` operator to check for meshtastic existence safely.
This allows the overlay to work with both stable (25.11) and unstable.
meshtastic package doesn't exist in nixpkgs 25.11 stable.
Use optionalAttrs to skip the override when not available,
allowing the flake to build against both stable and unstable.