`arboard` [doesn't support support iOS
yet](https://github.com/1Password/arboard/pull/103), so this PR adds iOS
to the conditions that prevent `arboard` from being enabled.
Launching an app on a physical device results in a long timeout (~8s)
while trying to connect to the X11 server (the timeout is immediate when
launching on a simulator), with the following trace:
```
egui_winit:📋 Failed to initialize arboard clipboard: Unknown error while interacting with the clipboard: X11 server connection timed out because it was unreachable
```
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