feat: add set-nix-channel module to manage nix-channel automatically

utils/modules/set-nix-channel.nix

@@ -0,0 +1,40 @@
+# utils/modules/set-nix-channel.nix
+{ config, pkgs, ... }:
+
+{
+  systemd.services.set-nix-channel = {
+    description = "Set nix-channel from /var/bento/channel";
+    after = [ "network.target" ]; # Ensure network is up
+    wantedBy = [ "multi-user.target" ]; # Start on boot
+
+    serviceConfig = {
+      Type = "oneshot";
+      RemainAfterExit = true; # Consider the service "active" after successful run
+      User = "root";
+      ExecStart = pkgs.writeShellScript "set-nix-channel-script" ''
+        set -e # Exit immediately if a command exits with a non-zero status.
+        CHANNEL_FILE="/var/bento/channel"
+        CHANNEL_NAME="nixos"
+
+        if [ ! -f "$CHANNEL_FILE" ]; then
+          echo "Error: Channel file $CHANNEL_FILE not found." >&2
+          exit 1
+        fi
+
+        CHANNEL_URL=$(cat "$CHANNEL_FILE")
+
+        if [ -z "$CHANNEL_URL" ]; then
+          echo "Error: Channel file $CHANNEL_FILE is empty." >&2
+          exit 1
+        fi
+
+        # Attempt to remove existing channel to ensure clean state, ignore error if it doesn't exist
+        ${pkgs.nix}/bin/nix-channel --remove "$CHANNEL_NAME" || true
+        ${pkgs.nix}/bin/nix-channel --add "$CHANNEL_URL" "$CHANNEL_NAME"
+        ${pkgs.nix}/bin/nix-channel --update "$CHANNEL_NAME"
+
+        echo "Successfully set and updated channel '$CHANNEL_NAME' from $CHANNEL_FILE."
+      '';
+    };
+  };
+}