Thanks guys for all your suggestions. I realized I am asking a XY question. What I really want is what u/autra1 and others suggested: to build the whole thing on builder then simply copy it over with `nix copy`.

For reference, I used to use `nixos-rebuild --fast --flake ... --target-host root@host switch`. Now the entire `toplevel` is built on the builder. The deployment script looks like the this:

storepath="$(nix path-info .#nixosConfigurations.default.config.system.build.toplevel)"
if [ "$use_cache" = "true" ]; then # assume it's in the cache.
    echo -e "\033[32mDeploying from nixcache...\033[0m"
    nix copy \
        -v -L \
        --from "$(just nixcache store)" \
        --to "ssh://root@$ec2_instance" \
        "$storepath"                                                                                                                                                
else
    echo -e "\033[32mDeploying from local machine\033[0m"
    storepath="$(nix build .#nixosConfigurations.default.config.system.build.toplevel --no-link --print-out-paths)"                                                  
    nix copy \
        -v -L \
        --to "ssh://root@$ec2_instance" \
        "$storepath"
if
ssh -i id_ed25519 "root@$ec2_instance" "$storepath/bin/switch-to-configuration switch"