[desktop] Provide a list of environment variables to control which backends are used

[jeromelaban]

What would you like to be added

The ability to control X11/Framebuffer on linux using environment variables, to force framebuffer even if X11 is available.

Why is this needed

Local configuration without having to rebuild

For which platform

Skia (Linux Framebuffer)

Anything else we need to know?

No response

[ramezgerges]

Shouldn't this be controlled in the SkiaHostBuilder instead? i.e.

SkiaHostBuilder.Create()
				...
				.UseX11()
				.UseLinuxFrameBuffer()
```				
should use X11 if available and fall back to FB, while

SkiaHostBuilder.Create() ... .UseLinuxFrameBuffer() .UseX11()

should do the opposite, no?