3rd party kernel modules does not work with module signing

We sign kernel modules since #14826 (closed), but we don't sign 3rd party modules.

We need to have a way to configure and store private key on the builders which can be re-used for kernel module signing. We could for example have a kernel key stored together with the abuild key or similar, and have an KERNEL_SIGNING_KEY="$HOME/.abuild/kernel_signing_key.pem" in ~/.abuild/abuild.conf

ref: https://www.kernel.org/doc/html/v6.1/admin-guide/module-signing.html

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