fix: add back required TARGETARCH for installer

Adds back in the required TARGETARCH for installer so extensions can be built off installer again as nvidia nonfree extension building was broken.

Fixes: #7155
Refs: #7115

Signed-off-by: Michael A. Davis <6325127+mrmichaeladavis@users.noreply.github.com>
Signed-off-by: Andrey Smirnov <andrey.smirnov@talos-systems.com>
(cherry picked from commit d4e94f7)

Dockerfile

@@ -707,6 +707,8 @@ LABEL org.opencontainers.image.source https://github.com/siderolabs/talos
 ENTRYPOINT ["/bin/installer"]
 
 FROM installer-image-squashed AS installer
+ARG TARGETARCH
+ENV TARGETARCH ${TARGETARCH}
 ONBUILD RUN apk add --no-cache --update \
     cpio \
     squashfs-tools \