ci: install zstd on s390x runners

zstd is installed by [default in GH
runners](https://github.com/actions/runner-images/blob/main/images/linux/Ubuntu2004-Readme.md).

Having it by default, we can start leveraging it when uploading
artifacts. It has a better compression ratio and is multithreaded.

Signed-off-by: Manu Bretelle <chantr4@gmail.com>

ci/rootfs/s390x-self-hosted-builder/actions-runner-libbpf.Dockerfile

@@ -25,6 +25,7 @@ RUN apt-get update && apt-get -y install \
         software-properties-common \
         sudo \
         tree \
+        zstd \
         iproute2 \
         iputils-ping