Fix: Use dynamic Python command to run uvicorn and support pyenv setups.

Updated the uvicorn startup command to dynamically detect and use either 'python3' or 'python',
ensuring compatibility across different environments and pyenv setups.
This avoids issues where only one of the commands is available.

backend/start.sh

@@ -65,4 +65,6 @@ if [ -n "$SPACE_ID" ]; then
   export WEBUI_URL=${SPACE_HOST}
 fi
 
-WEBUI_SECRET_KEY="$WEBUI_SECRET_KEY" exec uvicorn open_webui.main:app --host "$HOST" --port "$PORT" --forwarded-allow-ips '*' --workers "${UVICORN_WORKERS:-1}"
+PYTHON_CMD=$(command -v python3 || command -v python)
+
+WEBUI_SECRET_KEY="$WEBUI_SECRET_KEY" exec "$PYTHON_CMD" -m uvicorn open_webui.main:app --host "$HOST" --port "$PORT" --forwarded-allow-ips '*' --workers "${UVICORN_WORKERS:-1}"