dockerfile: Install the zip tool in the base image

This is needed when packaging the cross built toolchains anyway.

Dockerfile.cross

@@ -11,11 +11,6 @@
 ARG BASE=mstorsjo/llvm-mingw:dev
 FROM $BASE
 
-RUN apt-get update -qq && \
-    apt-get install -qqy zip && \
-    apt-get clean -y && \
-    rm -rf /var/lib/apt/lists/*
-
 ARG CROSS_ARCH=x86_64
 ENV CROSS_TOOLCHAIN_PREFIX=/opt/llvm-mingw-$CROSS_ARCH