image-builder: --no-cache instead of CACHEBUST arg

CACHEBUST only invalidates the umoci layer; the broken cached layer on
the Oracle runner is upstream (apt-install), so step 10 still tried to
exec /bin/sh from a corrupt parent FS. --no-cache rebuilds every layer
fresh — slower (~5 min apt re-run per build) but reliable on every
runner regardless of buildkit storage state.

build_image.sh

@@ -355,7 +355,7 @@ fi
 
 # --- Step 2: Build distribution image ---
 run_stage "Build image builder image" \
-    docker build --pull --build-arg "CACHEBUST=$(date +%s)" \
+    docker build --pull --no-cache \
         -t ps5-image-builder -f "$SCRIPT_DIR/docker/image-builder/Dockerfile" "$SCRIPT_DIR"
 
 if [ "$DISTRO" = "all" ]; then

docker/image-builder/Dockerfile

@@ -35,13 +35,7 @@ RUN mkdir -p /root/go/src/github.com/lxc/ && \
 RUN apt-get update && apt-get install -y --no-install-recommends \
     skopeo \
     && rm -rf /var/lib/apt/lists/*
-# CACHEBUST: one of the org runners has a broken cached layer for the umoci
-# install that fails with `/bin/sh: no such file`. Pass a fresh value via
-# `docker build --build-arg CACHEBUST=$(date +%s)` to force rebuild of this
-# step (the apt installs above stay cached).
-ARG CACHEBUST=0
-RUN echo "cachebust=$CACHEBUST" && \
-    go install github.com/opencontainers/umoci/cmd/umoci@v0.5.0 && \
+RUN go install github.com/opencontainers/umoci/cmd/umoci@v0.5.0 && \
     cp /root/go/bin/umoci /usr/local/bin/umoci && \
     rm -rf /root/go/pkg