Fix o3de CLI install on read-only /app via prebuilt sdist

The real first-launch installer is python/get_python.sh, not LYPython.cmake.
It does `pip install -e scripts/o3de`, whose egg_info write hits read-only
/app and fails ([Errno 30]). get_python.sh has a built-in immutable-install
hook: if scripts/o3de/dist/o3de-1.0.0.tar.gz exists it installs that
(non-editable) instead. Pre-build that tarball at Flatpak build time
(python3 setup.py sdist) so the runtime CLI lands in the writable ~/.o3de
venv. Verified end-to-end: tarball install -> `import o3de` resolves from
site-packages.

Also patch every LYPython.cmake copy (not just the first) for the separate
project-build CMake-configure path, and add python3/python3-setuptools to CI.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitea/workflows/build-flatpak.yml

@@ -28,6 +28,7 @@ jobs:
           apt-get update
           DEBIAN_FRONTEND=noninteractive apt-get install -y --no-install-recommends \
             ca-certificates curl git jq xz-utils zstd binutils tar \
+            python3 python3-setuptools \
             flatpak
 
       # Done as a plain git clone instead of actions/checkout@v4: the bare