FIX: build-disk workflow

.github/workflows/build-disk.yml

@@ -23,6 +23,7 @@ env:
   IMAGE_REGISTRY: "ghcr.io/${{ github.repository_owner }}"  # do not edit
   DEFAULT_TAG: "latest"
   BIB_IMAGE: "ghcr.io/lorbuschris/bootc-image-builder:20250608" # "quay.io/centos-bootc/bootc-image-builder:latest" - see https://github.com/osbuild/bootc-image-builder/pull/954
+  ROOTFS: ext4
 
 concurrency:
   group: ${{ github.workflow }}-${{ github.ref || github.run_id }}
@@ -79,6 +80,7 @@ jobs:
           image: ${{ env.IMAGE_REGISTRY }}/${{ env.IMAGE_NAME }}:${{ env.DEFAULT_TAG }}
           chown: ${{ env.USER_UID }}:${{ env.USER_GID }}
           types: ${{ matrix.disk-type }}
+          rootfs: ${{ env.ROOTFS }}
           additional-args: --use-librepo=True
 
       - name: Upload disk images and Checksum to Job Artifacts