It would appear that the following command builds entirely remotely, but, alas, this takes 100% local CPU (nix eval):
sudo --preserve-env=NIX_SSHOPTS \
nixos-rebuild \
--verbose \
--show-trace \
--use-remote-sudo \
--no-build-nix \
--build-host "${TARGET}" \
--target-host "${TARGET}" \
--flake "${FLAKE}" \
boot
What am I doing wrong? (the build succeeds, though)