nlyy: did you try just pinentry-emacs rather than nixos.pinentry-emacs?
```
$ nix-shell -p pinentry-emacs --command 'which pinentry-emacs'
/nix/store/6npsbnnj7c0cbamnwsmg0cs675kg8w3w-pinentry-1.1.0-emacs/bin/pinentry-emacs
```