Fixes: #23159

Context: dotnet/runtime#4538 (comment)
Context: https://github.com/unoplatform/uno/blob/ffbb0c0ed47679ede7c67effaa979e7a9934ed33/doc/articles/controls/WebView.md?plain=1#L198-L206

Use of WebView2 on Linux Wayland requires that the `GDK_BACKEND`
environment variable be set, lest Bad Things Happen™.

On the ignorable side, the bad things are simply messages written
to stdout.  For example, build and run SamplesApp.Skia.Generic:

	dotnet build -c Release -f net10.0 src/SamplesApp/SamplesApp.Skia.Generic/SamplesApp.Skia.Generic.csproj
	LD_PRELOAD=/usr/lib/aarch64-linux-gnu/libfreetype.so.6.20.1 \
	  dotnet src/SamplesApp/SamplesApp.Skia.Generic/bin/Release/net10.0/SamplesApp.Skia.Generic.dll

(The $LD_PRELOAD is needed to avoid [a *different* problem][0], which
should get fixed by updating SkiaSharp; see #23598.)

Once SamplesApp.Skia.Generic has launched:

 1. Click the "checkbox list" icon on the top-left to show the
    RuntimeTests panel.
 2. In the "Enter test filter here" textbox, enter
    `When_ExecuteScriptAsync`.
 3. Click the ▶️ Run button

Console output will contain messages such as:

	(SamplesApp.Skia.Generic:25303): Gdk-CRITICAL **: 09:20:32.946: gdk_x11_window_get_xid: assertion 'GDK_IS_X11_WINDOW (window)' failed
	…
	fail: Uno.UI.WebView.Skia.X11.X11NativeWebView[0]
	      SetScrollingEnabled is not supported on the X11 target.
	…
	warn: Uno.WinUI.Runtime.Skia.X11.X11ApplicationHost[0]
	      X11 protocol error — ErrorCode: 3, RequestCode: X_UnmapWindow, MinorCode: 0, ResourceId: 0x0, Serial: 428

However, the tests *pass* and the app *doesn't crash*.

Enter #23159: if we build Uno.Gallery for *Native AOT* and hit the
WebView2 logic, we see the same `Gdk-CRITICAL` message, *and also*
these messages:

	fail: Uno.WinUI.Runtime.Skia.X11.X11NativeOverlappedPresenter[0]
	      Couldn't get OverlappedPresenterState: _NET_WM_STATE does not exist on the window. Make sure you use an EWMH-compliant WM.
	fail: Uno.WinUI.Runtime.Skia.X11.X11NativeOverlappedPresenter[0]
	      Couldn't get OverlappedPresenterState: _NET_WM_STATE does not exist on the window. Make sure you use an EWMH-compliant WM.

followed by a *hard crash*:

	(Uno.Gallery:8586): Gdk-CRITICAL **: 14:46:16.851: gdk_x11_window_get_xid: assertion 'GDK_IS_X11_WINDOW (window)' failed
	X Error of failed request:  BadWindow (invalid Window parameter)
	  Major opcode of failed request:  3 (X_GetWindowAttributes)
	  Resource id in failed request:  0x0
	  Serial number of failed request:  103
	  Current serial number in output stream:  104

The workaround for this, as documented in
`doc/articles/controls/WebView.md`, is to export the `GDK_BACKEND`
environment variable to `x11`:

	export GDK_BACKEND=x11
	dotnet …

This works, but requires reading documentation (who does that?!).

The ideal fix would be to have `X11NativeWebProvider` export the
environment variable during app startup.  The problem is that the
"obvious" way to do this:

	Environment.SetEnvironmentVariable("GDK_BACKEND", "x11");

[*does not work*][1]

> we will not support the case where a user wants to modify the
> environment from managed code and expects to see those changes
> reflected in a native library they PInvoke to, or vice versa.

Since we *need* native code in `libgdk*` et al to see the
`GDK_BACKEND` environment variable, we can't use
`Environment.SetEnvironmentVariable()` to do so.

Instead, P/Invoke into **setenv**(3).  This allows us to set the
`GDK_BACKEND` environment variable on the native side, allowing
`libdk*` et al to see the value we set and behave accordingly.

With this fix in place, `SamplesApp.Skia.Generic.dll` no longer
produces the `Gdk-CRITICAL` or X11 protocol error messages.

[0]: #20532 (comment)
[1]: dotnet/runtime#4538 (comment)