Create VM image .zip files in parallel using pigz (#4067)

When creating OVA image, the CPU is slacking at the end of the build because it
is creating three ZIP archives, each one on a single CPU only. As we're
creating only single-entry archives, we can use pigz to use all cores.

The actual speedup on my machine (16C/32T) reflects the number of cores - it
takes around 2 seconds instead of 1 minute.

Dockerfile

@@ -38,6 +38,7 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
         openssh-client \
         patch \
         perl \
+        pigz \
         python3 \
         python3-matplotlib \
         python-is-python3 \

buildroot-external/scripts/hdd-image.sh

@@ -100,5 +100,5 @@ function convert_disk_image_zip() {
     hdd_img="$(hassos_image_name "${hdd_ext}")"
 
     rm -f "${hdd_img}.zip"
-    zip -j -m -q -r "${hdd_img}.zip" "${hdd_img}"
+    pigz -q -K -S ".zip" "${hdd_img}"
 }