git/buildGoModule + private repositories

I worked around this in GitHub by doing:

    - run: |
        eval $(ssh-agent -s)
        ssh-add - <<< "${MACHINE_USER_KEY}"
        nix build -L .#image
      env:
        MACHINE_USER_KEY: ${{secrets.MACHINE_USER_PAT }}

This allows the cloning of private git repos that use the git transport.