Add builder workflow to paths triggering rebuild on merge (#6691)

When only the workflow changes, CI doesn't trigger rebuild when the PR
is merged to the `main` branch. Since it's a legitimate build trigger,
add it to the paths list.

.github/workflows/builder.yml

@@ -25,6 +25,7 @@ on:
   push:
     branches: ["main"]
     paths:
+      - ".github/workflows/builder.yml"
       - "rootfs/**"
       - "supervisor/**"
       - Dockerfile