### Purpose
Locally, on Windows 11, and on the windows-2025 GitHub runner (go 1.23
and 1.24), the `TestCopyRange` test is failing with `The request is not
supported.`
On windows-2022 and windows-2019:
```go
err == syscall.ENOTSUP
```
worked, but on Windows 11 and windows-2025, we need:
```go
errors.Is(err, errors.ErrUnsupported)
```
### Testing
Tested on Windows 11, windows-2019, windows-2022, and
[windows-2025](https://github.com/rasa/syncthing/actions/runs/15525123437/job/43703630634#step:7:2811).
This allows users to easily disable nightly builds in their forks,
simply by disabling the
build-nightly action.
### Testing
I tested it in my fork, and it works.
### Purpose
This fixes#9775. I also improved the comments as they were lacking.
My apologies for introducing this bug. In summary, the bug was
```
mode = mode ^ (ModeSymlink | ModeIrregular)
```
didn't correctly reset those bits. This correctly resets them:
```
mode = mode &^ ModeSymlink &^ ModeIrregular
```
Tested and working in Windows 11 version 10.0.22631.4317. I didn't test
in other versions, but I'm sure this is the only issue.
Ross Smith II
