emacsPackages.mozc: fix mozc_emacs_helper path

The substitution in melpa-packages.nix rewrote `"mozc_emacs_helper"` in
`mozc.el` to `${pkgs.ibus-engines.mozc}/lib/mozc/mozc_emacs_helper`,
but that path no longer exists.

Since the Bazel-based rewrite and the optional ibus split in #483843,
`mozc_emacs_helper` is installed at `$out/bin/` of the base `mozc` package,
while `$out/lib/mozc/` only contains `mozc_server`,
`mozc_renderer`, and `mozc_tool`.
As a result, `mozc.el` invoked a non-existent helper and `mozc-mode` failed to start with

> 変換エンジンプログラムの起動に失敗しました。

Switch the reference to `pkgs.mozc` so it points at the actual binary
location and avoids pulling in the IBus front-end for Emacs users.

Assisted-by: Claude Code (claude-opus-4-7)

pkgs/applications/editors/emacs/elisp-packages/melpa-packages.nix

@@ -773,7 +773,7 @@ let
           mozc = super.mozc.overrideAttrs (attrs: {
             postPatch = attrs.postPatch or "" + ''
               substituteInPlace src/unix/emacs/mozc.el \
-                --replace '"mozc_emacs_helper"' '"${pkgs.ibus-engines.mozc}/lib/mozc/mozc_emacs_helper"'
+                --replace '"mozc_emacs_helper"' '"${pkgs.mozc}/bin/mozc_emacs_helper"'
             '';
           });