ci: install wget package on s390x runners

`wget` is installed by default in GH runners.
It is used in
[`get-linux-source`](https://github.com/libbpf/ci/blob/79c799d6fb31071c139bafa69639501b6b8ab76b/get-linux-source/checkout_latest_kernel.sh#L32)
to download source faster than through a git fetch.

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

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

@@ -16,6 +16,7 @@ RUN apt-get update && apt-get -y install \
         cmake \
         cpu-checker \
         curl \
+        wget \
         flex \
         git \
         jq \