kata-deploy: tools: tests: Use zstd instead of xz

Although the compress ratio is not as optimal as using xz, it's way
faster to compress / uncompress, and it's "good enough".

This change is not small, but it's still self-contained, and has to get
in at once, in order to help bisects in the future.

Signed-off-by: Fabiano Fidêncio <fabiano@fidencio.org>

tools/osbuilder/rootfs-builder/alpine/Dockerfile.in

@@ -26,6 +26,7 @@ RUN apk update && apk add --no-cache \
     musl-dev \
     protoc \
     tar \
-    xz
+    xz \
+    zstd
 # aarch64 requires this name -- link for all
 RUN ln -s /usr/bin/gcc "/usr/bin/$(uname -m)-linux-musl-gcc"