build libstxxl in release mode, fixes #30437

This fixes the issues raised in #30437. The issue was caused by ` *std_cmake_args` which defines `-DCMAKE_BUILD_TYPE=NONE`. Explicitly setting to `-DCMAKE_BUILD_TYPE=Release` produces a proper release binary.
So: remove the -DCMAKE_BUILD_TYPE=None from the list and then add Release. Ideally, though, we'd work out why this happens and fix that as we set `None` so we can set CFLAGS ourselves.