CI: install cmake/python3 for O3DE's get_python.sh

get_python.sh runs on the host shell and shells out to cmake to fetch the
engine's Python runtime, but the bare ubuntu container had neither cmake
nor python3. Add them (and tar) to the dependency install.

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

README.md

@@ -87,7 +87,8 @@ the upstream version has no matching `vX.Y.Z` tag yet, so reruns are cheap no-op
 ## Building locally
 
 ```sh
-sudo apt install flatpak      # or your distro's equivalent
+# flatpak + cmake (cmake is used by O3DE's get_python.sh) + curl
+sudo apt install flatpak cmake      # or your distro's equivalent
 ./scripts/build.sh
 ```