gopass: Support passAlias parameter in derivation

Add a `passAlias` to `gopass` derivation, similarly to how there's `viAlias` parameter supported by vim/neovim derivations.
Apart from convenience this is also necessary for e.g. making `docker-credential-helpers` work without patching the latter - the package uses `pass` executable from `PATH` https://github.com/docker/docker-credential-helpers/blob/f78081d1f7fef6ad74ad6b79368de6348386e591/pass/pass_linux.go#L67. By setting `passAlias` to `true` (e.g. via overlays like so: https://github.com/rvolosatovs/infrastructure/blob/94971f34db9df79b6666e626fa52840eb1fee840/nixpkgs/overlays.nix#L315-L317), secure Docker authentication via `gopass` "just works"™