ci: fix codspeed segfault (#35502)

Pin Python to 3.13.11 in the CodSpeed benchmark job. CPython 3.13.12
introduced a segfault in CodSpeed's walltime callgraph post-processing
([CodSpeedHQ/pytest-codspeed#106](https://github.com/CodSpeedHQ/pytest-codspeed/issues/106)),
causing all `libs/core` benchmark runs to fail with `exit status: 139`
since Feb 26 despite all 13 benchmarks passing.