Fix read-only /app failure + add app icon

On first launch O3DE pip-installs its 'o3de' CLI with 'pip install -e'
(editable), which writes an egg-info next to the source under read-only
/app and fails ([Errno 30] Read-only file system), leaving the venv broken
("unable to install O3DE's built-in Python").

- Patch cmake/LYPython.cmake at build time to force a normal (non-editable)
  install, which builds in a temp dir and lands in the writable ~/.o3de venv.
- Drop the build-time get_python "bake": the venv is per-user (keyed to
  $HOME), so it can only be created at runtime; baking under a throwaway
  build HOME did nothing. Speeds up CI; removes cmake/python3 build deps and
  the network build-arg.
- Add a real launcher icon (org.o3de.O3DE.png).

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

.gitea/workflows/build-flatpak.yml

@@ -26,11 +26,9 @@ jobs:
       - name: Install build dependencies
         run: |
           apt-get update
-          # cmake + python3: O3DE's get_python.sh (run on the host, not in the
-          # sandbox) uses cmake to fetch its Python runtime.
           DEBIAN_FRONTEND=noninteractive apt-get install -y --no-install-recommends \
             ca-certificates curl git jq xz-utils zstd binutils tar \
-            flatpak cmake python3
+            flatpak
 
       # Done as a plain git clone instead of actions/checkout@v4: the bare
       # ubuntu image has no Node.js, so JavaScript actions fail with exit 127.