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>

.gitea/workflows/build-flatpak.yml

@@ -26,9 +26,11 @@ 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 \
+            ca-certificates curl git jq xz-utils zstd binutils tar \
-            flatpak
+            flatpak cmake python3
 
       # 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.

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
 ```