nixos/frigate: retain the runtime state file

Some of the runtime knobs cannot be replicated well with file-based
configuration. A prime example of this is soft-disabling a camera (I
have my 3D printer camera in frigate, why are you looking at me like
that 😅)

In short, `camera.enabled: false` in config will hard-disable the camera
instead and it cannot be re-enabled either via UI or MQTT. The UI/MQTT
toggle, however does remember the camera state across restarts and will
restore it when it starts back up, making it a soft-disable mechanism.

This state is stored in the file next to the config without a way to
move it. We also can't use a Bind mount or symlink for this file,
because frigate utilizes the atomic-move pattern to atomically modify
this file, which doesn't work with bind mounts and will overwrite the
symlink.

nixos/modules/services/video/frigate.nix

@@ -733,7 +733,10 @@ in
         "multi-user.target"
       ];
       environment = {
-        CONFIG_FILE = "/run/frigate/frigate.yml";
+        # Frigate keeps some of its persistent runtime state in a file next to the configuration,
+        # so the config must also live in persistent storage (even though we overwrite it on every
+        # startup.)
+        CONFIG_FILE = "/var/lib/frigate/frigate.yml";
         HOME = "/var/lib/frigate";
         PYTHONPATH = cfg.package.pythonPath;
       }
@@ -762,7 +765,7 @@ in
             ${lib.getExe pkgs.findutils} /var/cache/frigate -not -path '/var/cache/frigate/model_cache/*' -type f -delete
           '')
           (pkgs.writeShellScript "frigate-create-writable-config" ''
-            cp --no-preserve=mode ${configFile} /run/frigate/frigate.yml
+            cp --no-preserve=mode ${configFile} "$CONFIG_FILE"
           '')
         ]
         ++ lib.optionals (!config.systemd.services.frigate.environment ? LIBAVFORMAT_VERSION_MAJOR) [