fix(transformers): use Python 3.12

SciPy 1.18 requires Python 3.12 or newer. Keep this backend on a
compatible portable Python for Linux and macOS builds.

Assisted-by: Codex:gpt-5.6 [Codex]

Author: localai-org-maint-bot

backend/python/transformers/install.sh

@@ -1,6 +1,9 @@
 #!/bin/bash
 set -e
 
+PYTHON_VERSION="3.12"
+PYTHON_PATCH="11"
+
 backend_dir=$(dirname $0)
 if [ -d $backend_dir/common ]; then
     source $backend_dir/common/libbackend.sh