Run just-built Python with -s -S

Add `-s` and `-S` flags to `PYTHON_FOR_BUILD` to ensure that the new
executable does not access the system site-packages.  This avoids
sandbox errors when upgrading from 3.13.0b1.

Bug: https://bugs.gentoo.org/934079

configure.ac

@@ -170,7 +170,7 @@ AC_ARG_WITH([build-python],
     AS_VAR_IF([cross_compiling], [yes],
       [AC_MSG_ERROR([Cross compiling requires --with-build-python])]
     )
-    PYTHON_FOR_BUILD='./$(BUILDPYTHON) -E'
+    PYTHON_FOR_BUILD='./$(BUILDPYTHON) -E -s -S'
     PYTHON_FOR_FREEZE="./_bootstrap_python"
   ]
 )