Autodetected toolchain on Windows with Visual Studio 2026 issues a warning due to `/DEBUG:FASTLINK`

`/DEBUG:FASTLINK` is no longer supported in VS 2026. To prevent a warning, the autodetection machinery should not use `FASLINK` in that situation.

```
LINK : warning LNK4315: /DEBUG:FASTLINK is no longer supported. Using /DEBUG:FULL instead. Use a VS 2022 toolchain to continue building with /DEBUG:FASTLINK
```