haven used nixos cache in a while, does anyone know whats wrong with my command ?
```
nix copy --from https://cache.nixos.org --trusted-public-keys 'hydra.nixos.org-1:CNHJZBh9K4tP3EKF6FkkgeVYsS3ohTl+oS0Qa8bezVs=' /nix/store/l86d2y5bz3fg6yjc6lz400hvz18wi71i-uboot-rock64-rk3328_defconfig-2020.01 

error: cannot add path '/nix/store/l86d2y5bz3fg6yjc6lz400hvz18wi71i-uboot-rock64-rk3328_defconfig-2020.01' because it lacks a valid signature
```