kernel-build: use mirror when checking out the source tree

[theihor]

When checking out the linux source, look at REFERENCE_REPO_PATH and pass it as --reference to git clone.

This caching will only work for jobs running on our custom runners.

A couple of caveats:

• As it turns out, the main BPF CI jobs cloned Linux source with actions/checkout, which does not have a way of passing --reference option. This is why in this change actions/checkout is replaced with custom git clone command. Without that, there is no point in adding the mirror to runner images.
• s390x VMs don't have enough storage for this :( Fortunately, this isn't a big deal, because we cross-compile on x86_64 anyways.
  • https://github.com/kernel-patches/bpf/actions/runs/18569955622
  • s390x: don't set up Linux mirror runner#88

It would be nice to consolidate Linux source tree cloning and drop inputs.download_sources flag, but that requires some thought. Maybe next time.

See also:

• Build-in a Linux source mirror repository runner#87
• libbpf/ci@3d86310