Regression on TARGETARG

[bonitao]

Minimal repro:

# syntax = devthefuture/dockerfile-x:v1.3.4
FROM alpine
ARG TARGETARCH
RUN echo $TARGETARCH > /root/targetarch.txt
CMD [ "cat" , "/root/targetarch.txt" ]

When running this container, instead of amd64, I get the empty string inserted in /root/targetarch.txt. If instead I use devthefuture/dockerfile-x:v1.3.3 (the previous version), then everything goes as expected.