Put zenity to flatpak /usr/bin folder

I try to create a flatpak application for [Instant meshes](https://github.com/wjakob/instant-meshes) in particular the manifest is the following:

id: org.flatpak.InstantMeshes
runtime: org.gnome.Platform
runtime-version: "44"
sdk: org.gnome.Sdk
command: InstantMeshes
desktop-file-name-suffix: " (Nightly)"
finish-args:
  - --share=ipc
  - --socket=x11
  - --socket=wayland
  - --filesystem=home
  - --device=dri
modules:
  - name: zenity
    buildsystem: meson
    sources:
      - type: archive
        url: https://download.gnome.org/sources/zenity/3.41/zenity-3.41.0.tar.xz 
        sha256: 19b676c3510e22badfcc3204062d432ba537402f5e0ae26128c0d90c954037e1
  
  - name: build instant meshes
    buildsystem: cmake
    build-commands:
      - cmake -DCMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX=/app
      - make -j 4
      - install -D "./Instant Meshes" /app/bin/InstantMeshes
    sources:
      - type: git
        url: https://github.com/wjakob/instant-meshes.git 
        branch: master
    build-options:
      append-path: "/usr/lib/"

  - name: Intant Meshes
    buildsystem: simple
    build-commands:
      # - install /app/bin/zenity /usr/bin/zenity
      - install -D InstantMeshes.desktop /app/share/applications/org.flatpak.InstantMeshes.desktop
      - install -D instantmeshes.png /app/share/icons/hicolor/256x256/apps/instantmeshes.png
    ensure-writable:
      - /app/bin/zenity
    sources:
      - type: file
        path: InstantMeshes.desktop
      - type: file
        path: instantmeshes.png

But at runtime get an error when try to open folders:

sh: line 1: /usr/bin/zenity: File o directory do not exist

That because zenity is in /app/bin path, but it is read only and I cannot modify from basic flatpak manifest. There are any posibilities to instroduce zenity into /usr/bin folder? Probably with runtimes?