ISO: pass --rootfs btrfs (image carries no default rootfs type)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.forgejo/workflows/build-iso.yml

@@ -37,6 +37,7 @@ jobs:
             --mount type=volume,src=kamos-buildah,dst=/var/lib/containers/storage,volume-subpath=storage \
             quay.io/centos-bootc/bootc-image-builder:latest \
             --type anaconda-iso \
+            --rootfs btrfs \
             "${REGISTRY_HOST}/${OWNER}/${{ inputs.variant }}:stable"
 
       - name: Extract ISO from volume