lisa._kmod: Avoid broken split .BTF section in .ko

FIX

Fix the issue reported at [1] by removing the vmlinux from the kernel
tree before building modules, so that Kbuild will not be tempted to
create a broken split .BTF section.

[1] https://lore.kernel.org/all/YfK18x%2FXrYL4Vw8o@syu-laptop/t/#md877c45455918f8c661dc324719b91a9906dc7a3