fix: reproducible JS builds in rock OCI

Author: edisile

rockcraft.yaml

@@ -23,7 +23,7 @@ parts:
       set -eux
       # install dependencies
       npm install -g yarn
-      yarn install
+      yarn install --immutable
       # build the UI
       yarn run build
       mkdir -p "$CRAFT_PART_INSTALL/flask/app"