The problem is introduced by the guix update on fd885160fcfde710e415252463c1034f22f88e59.
Reverting this commit makes the problem go away, so a bisect on the guix package version is feasible. The problem can be triggered in the installer image by doing 'guix build guix' in a root terminal. The first attempt fails, while subsequent attempts succeed. rm -rf /var/guix/substitute triggers the error again. -- OpenPGP Key Fingerprint: 7988:3B9F:7D6A:4DBF:3719:0367:2506:A96C:CF63:0B21
