Fix libbpf submodule checkout in bpftool

[tpapagian]

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Now in the Dockerfile we do:
git clone --recurse-submodules https://github.com/libbpf/bpftool.git . && git checkout ${BPFTOOL_REV} which means that we fetch the libbpf submodule version of the main branch of bpftool and then we move to the appropriate commit that we need. After the checkout we also need to fetch the correct version of the submodules. This patch fixes that.

Fix libbpf submodule checkout in bpftool when building the tetragon image.

[qmonnet]

This is the way