Ensure zstd is available in DEB build container

The bundled fpm uses tar --use-compress-program zstd, which
requires the zstd binary to be installed on the builder.

Made-with: Cursor

Author: emlowe

.github/workflows/build-linux-installer-deb.yml

@@ -176,6 +176,12 @@ jobs:
           cd ./build_scripts
           bash build_linux_deb-1-gui.sh
 
+      - name: Ensure zstd is available
+        run: |
+          if ! command -v zstd &> /dev/null; then
+            apt-get update && apt-get install --yes zstd
+          fi
+
       - name: Build .deb package
         run: |
           ldd --version