templates: make flake-parts input explicit

[MatthewCroughan]

Making the input implicit and relying on the registry has never worked for me, and I don't like relying on it

nix flake show
warning: Git tree '/home/matthew/tmp/llm-chain' is dirty
error:
       … while updating the lock file of flake 'git+file:///home/matthew/tmp/llm-chain'

       … while updating the flake input 'flake-parts'

       error: cannot find flake 'flake:flake-parts' in the flake registries

Each new template would just do this for me, and I always end up having to write the input explicitly anyway.

[roberth]

Thanks!