fix: add --compressed to chunkah args and clean buildah cache before rechunk

chunkah OCI archive (6.7 GiB uncompressed) did not fit on runner alongside
original image. --compressed uses zstd compression, reducing archive to ~3-4GB.
Also cleans buildah cache before rechunk to free additional space.

.github/workflows/build.yml

@@ -100,6 +100,9 @@ jobs:
           # Image-Config fuer chunkah sichern (Labels, Annotations, etc.)
           CONFIG=$(buildah inspect "$IMG" | jq -c .)
 
+          # Platz schaffen fuer den Rechunk (OCI-Archive + Original passen nicht beide)
+          sudo rm -rf /tmp/.buildah-cache/* 2>/dev/null || true
+
           # chunkah: content-based Layer-Splitting
           # --max-layers 128: Desktop-Images brauchen mehr als Default (64)
           # --prune /sysroot/: entfernt OSTree-Metadaten (optional seit bootc 1.1.3)
@@ -107,7 +110,7 @@ jobs:
           buildah build --skip-unused-stages=false \
             --from "$IMG" \
             --build-arg CHUNKAH_CONFIG_STR="$CONFIG" \
-            --build-arg "CHUNKAH_ARGS=--prune /sysroot/ --max-layers 128 --label ostree.commit- --label ostree.final-diffid-" \
+            --build-arg "CHUNKAH_ARGS=--prune /sysroot/ --max-layers 128 --compressed --label ostree.commit- --label ostree.final-diffid-" \
             -t "$CHUNKED_IMG" \
             https://github.com/coreos/chunkah/releases/download/v0.5.0/Containerfile.splitter