Add xxd to the base CI debian image

[sinkap]

xxd is used to generate a C header from a verification certificate (DER encoded) which is used to test BPF signing infrastructure in selftests.

[theihor]

@sinkap rootfs is not used by BPF CI (anymore). If xxd is a requirement for relevant BPF selftests, you need to add the package here: https://github.com/libbpf/ci/blob/main/run-vmtest/install-dependencies.sh#L11-L12

I know it's confusing, sorry :/

[theihor]

Just looked at your patch: https://lore.kernel.org/bpf/20250606232914.317094-13-kpsingh@kernel.org/

So xxd is required at selftests build time, not run time. Which means that the proper place for this dependency is https://github.com/libbpf/ci/blob/main/setup-build-env/install_packages.sh#L15-L20

I wonder if there is a way to generate that header without an additional dependency?

[sinkap]

I did not find an easy way to generate the header without xxd, it will be ugly and fragile, let's just use xxd.

I sent #193