Sets `AVALONIA_GLOBAL_SCALE_FACTOR` to the GNOME desktop scaling factor
based on
<https://github.com/AvaloniaUI/Avalonia/issues/9390#issuecomment-2382126451>,
falling back to the X FreeType DPI setting if the former is not
available.
Does not include `gsettings` and `xrdb` in build inputs, since these
should be available on the relevant platforms.
Bash does not support decimal/floating point arithmetic, so this *does*
include `bc` in the runtime dependencies.
59ed3fa2c4