Ben Gamari authored Mar 10, 2025 and Marge Bot committed Mar 19, 2025

As noted in #25838, previously `selectIOManager` failed to set
`rts_IOManagerIsWin32Native` in its `IO_MNGR_FLAG_AUTO`. This meant
that the MIO path was taken when WinIO was supposedly selected,
resulting in chaos.

Fixes #25838.