Normalize fork markers (#4623)
Looks much better than #4618:
```
DEBUG Pre-fork split universal took 0.644s
DEBUG Split python_version >= '3.12' and platform_machine == 'aarch64' and platform_system == 'Darwin' and platform_system == 'Linux' took 0.659s
DEBUG Split python_version == '3.9' and platform_machine == 'arm64' and platform_system == 'Darwin' took 0.291s
```
