Add flake.lock to be the list of items for automerge

Signed-off-by: Suguru Hirahara <did🔑z6MkvVZk1A3KBApWJXv2Ju4H14ErDfRGxh8zxdXSZ4vACDg5>

.github/renovate.json

@@ -32,6 +32,12 @@
 				"dock.mau.dev/mautrix/**"
 			],
 			"allowedVersions": "/^v0\\./"
+		},
+		{
+			"matchFileNames": [
+				"flake.lock"
+			],
+			"automerge": true
 		}
 	],
 	"pre-commit": {