jobs/build-podman-os: set rootfs-size: 0

Recent cosa change enabled osbuild based `secex` build,
where image contains more partitions (for dm-verity)
and `root` labeled filesystem is not the last one,
so now we should also set `rootfs-size`.

See also coreos/coreos-assembler@443b9da

Author: nikita-dubrovskii

jobs/build-podman-os.Jenkinsfile

@@ -41,7 +41,8 @@ def generate_diskvar_json(shortcommit, arch, artifacts, staging_repo, repo) {
             "container-imgref": "ostree-remote-registry:fedora:${repo}:5.1",
             "metal-image-size": "3072",
             "cloud-image-size": "10240",
-            "deploy-via-container": "true"
+            "deploy-via-container": "true",
+            "rootfs-size": "0"
         }
         """
         def file_path="./diskvars-${arch}-${artifact["suffix"]}.json"