But something like this would ofc work too

export STOREPATH=$(nix build --no-link --print-out-paths nixpkgs#hello)
nix copy $STOREPATH --to $somewhere
# run your YAML mangling bogus here

Easiest way actually is probably editing default.nix

            nix-csi = {
              namespace = "nix-csi";
              image = imageRef; # comment this out
              cache.storageClassName = "some nice CSI";
              authorizedKeys = [
                "YOUR KEY HERE :)"
              ];
              ctest = {
                enable = true;
                replicas = 1;
              };
            };

Then running

nix run --file . kubenixEval.deploymentScript -- --prune --yes