make sure pkgconfig is installed

Dockerfile

@@ -2,7 +2,9 @@ ARG RUNTIME=cpu
 FROM ghcr.io/mlverse/torch:${RUNTIME}-main-release-focal
 
 COPY DESCRIPTION DESCRIPTION
-RUN apt-get update -y && Rscript -e "try(pak::pak(lib=.Library))"
+RUN apt-get update -y && \
+    apt-get install -y pkg-config && \
+    Rscript -e "pak::pak(lib=.Library)"
 
 # This is to make sure we don't run into permission problems in HF Spaces
 # see: https://huggingface.co/docs/hub/spaces-sdks-docker#permissions