Example: https://hydra.nixos.org/build/254601361/nixlog/1/tail
Details:

++ chmod -R u+w /nix/store/wwxn8yz0376ailcnj18a0mbp9kf30r4z-lxc-5.0.3
chmod: changing permissions of '/nix/store/wwxn8yz0376ailcnj18a0mbp9kf30r4z-lxc-5.0.3/libexec/lxc/lxc-user-nic': Operation not permitted

The permissions

-rwsr-xr-x 1 nixbld1 nixbld 1271576 $date /nix/store/6g9fhcrc4iwcf4x4l1vsya15441n8lhi-lxc-5.0.3/libexec/lxc/lxc-user-nic

I'm baffled why this started, i.e. which commit triggered the change (can't see relevant change in setup hooks or coreutils), though it can surely get fixed even without knowing that. Introduced somewhere during staging* (PR #298548).

Note: obviously, nix will strip any set*id bits when registering the result into the nix store anyway, so the point is only to avoid the failure.