feat(iso): replace --clean-cache with --clean-build (cleans + rebuilds)

--clean-build clears all caches (Go, NVIDIA, lb packages, work dir)
and rebuilds the Docker image, then proceeds with a full clean build.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

iso/builder/build-in-container.sh

@@ -29,13 +29,14 @@ while [ $# -gt 0 ]; do
             AUTH_KEYS="$2"
             shift 2
             ;;
-        --clean-cache)
+        --clean-build)
             CLEAN_CACHE=1
+            REBUILD_IMAGE=1
             shift
             ;;
         *)
             echo "unknown arg: $1" >&2
-            echo "usage: $0 [--cache-dir /path] [--rebuild-image] [--authorized-keys /path/to/authorized_keys] [--clean-cache]" >&2
+            echo "usage: $0 [--cache-dir /path] [--rebuild-image] [--clean-build] [--authorized-keys /path/to/authorized_keys]" >&2
             exit 1
             ;;
     esac
@@ -50,8 +51,7 @@ if [ "$CLEAN_CACHE" = "1" ]; then
            "${CACHE_DIR:?}/lb-packages"
     echo "=== cleaning live-build work dir: ${REPO_ROOT}/dist/live-build-work ==="
     rm -rf "${REPO_ROOT}/dist/live-build-work"
-    echo "=== done, caches cleared ==="
-    exit 0
+    echo "=== caches cleared, proceeding with build ==="
 fi
 
 if ! command -v "$CONTAINER_TOOL" >/dev/null 2>&1; then