enable nested-virtualization and kvm

Dear salsa admins,

@josch filed a similar issue (#129 (closed)) on Dec 2018. It would be great to have kvm enable in the runner to have a better virtualization performance than tcg's. Things have changed since 2018, i.e. the containers are privileged now, and loading the kvm module should be possible. Also, I see gce supports nested virtualization.

Could you please enable nested virtualization and the kvm module in the shared runner?

Thanks in advance,

Santiago