nixos/bird: add services.bird*.checkConfig option
nixos/bird: add services.bird*.checkConfig option

This is useful when the config doesn't entirely live in the Nix store,
but is configured to include mutable config files written at runtime.

Co-Authored-By: Puck Meerburg <1c3dee6ab0ddac1f656e2d4cb5678203e9fa854a@puck.moe>

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