Update flake.lock, dropping unused nixpkgs input

On a7600ce all the
inputs on `flake.nix` were moved to `tests/flake.nix`, but `flake.lock`
was not updated correctly, and a reference to `nixpkgs` still remains,
producing this error upon evaluation when used as an input in my flake:

    evaluating file '<nix/derivation-internal.nix>'copying "/nix/store/xjzjf1f6nyg6yqxb6v5awf9hs14606yp-source" to the store...evaluating file '/nix/store/xjzjf1f6nyg6yqxb6v5awf9hs14606yp-source/flake.nix'evaluating file '/nix/store/xjzjf1f6nyg6yqxb6v5awf9hs14606yp-source/flake.nix'warning: updating lock file '"/nix/store/xjzjf1f6nyg6yqxb6v5awf9hs14606yp-source/flake.lock"':• Removed input 'nixpkgs'error:… while updating the lock file of flake 'path:/nix/store/xjzjf1f6nyg6yqxb6v5awf9hs14606yp-source?lastModified=0&narHash=sha256-dz303vGuzWjzOPOaYkS9xSW%2BB93PSAJxvBd6CambXVA%3D'error: opening file '/nix/store/xjzjf1f6nyg6yqxb6v5awf9hs14606yp-source/flake.lock': Read-only file system

I’ve thus updated `flake.lock` with the standard procedure:

```console
❯ nix flake lock --verbose
warning: updating lock file '"/home/pancho/sandbox/NixOS/nixos-hardware/flake.lock"':
• Removed input 'nixpkgs'
```

Author: panchoh