commands

[clever@amd-nixos:~]$ sudo socat UNIX-LISTEN:/tmp/hax,fork,mode=0070,group=nixbld UNIX-CLIENT:$SSH_AUTH_SOCK
[clever@amd-nixos:~]$ touch /tmp/ssh-config
[clever@amd-nixos:~/AEDB]$ nix-build default.nix -I ssh-auth-sock=/tmp/hax -I ssh-config-file=/tmp/ssh-config

default.nix

with import <nixpkgs> {};

fetchgitPrivate {
  url = "user@example.com:apps/repo/project_git";
  sha256 = "1jw7iapmkhz49wbv96jjy3x2qmdhz075q6haw6rzchgnd3vq1qp0";
}

it helps when the ssh-config file sets the following to prevent unexpected stopping:

cat << EOF > /tmp/ssh-config
Host *
    StrictHostKeyChecking=no
    UserKnownHostsFile /dev/null
EOF

i prefer to fix the HostKey problems via programs.ssh.knownHosts on nixos, it lets you pre-add the hostkeys system wide