build: improve robustness of fallback version extraction

Author: meltsufin

sdk-platform-java/.github/scripts/hermetic_library_generation.sh

@@ -129,7 +129,7 @@ if [[ "$current_branch" =~ ^release-please-- ]]; then
   echo "Detected release PR branch: $current_branch"
   if ! docker pull "${IMAGE_NAME}:${image_tag}"; then
     echo "Image not found for version ${image_tag}. Falling back to previous version from ${target_branch}."
-    previous_tag=$(git show "${target_branch}":.github/workflows/hermetic_library_generation.yaml | grep "image_tag:" | cut -d ':' -f 2 | cut -d '#' -f 1 | xargs)
+    previous_tag=$(git show "${target_branch}":.github/workflows/hermetic_library_generation.yaml | grep -m 1 "^[[:space:]]*image_tag:" | cut -d ':' -f 2- | cut -d '#' -f 1 | xargs)
     if [ -n "$previous_tag" ]; then
       echo "Using previous image version: $previous_tag"
       image_tag="$previous_tag"