If I'm not mistaken, this is because you need to add the builders to the 'kvm'
group.  From the manual ((guix) 2.4.1 Build Environment Setup):

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To use ‘guix system vm’ and related commands,
you may need to add the build users to the ‘kvm’ group so they can
access ‘/dev/kvm’, using ‘-G guixbuild,kvm’ instead of ‘-G guixbuild’
(*note Invoking guix system::).
--8<---------------cut here---------------end--------------->8---

-- 
Pierre Neidhardt
https://ambrevar.xyz/