appveyor: Add libxcvt build dep

Install libxcvt build dep on appveyor.

Explicitly install python3.8 lxml to ensure it matches python version
installed (to workaround issues with Cygwin installer).

Drop explicit configuration of hal and udev, as meson.build now knows to
turn those off for Cygwin.
5 jobs for windows-meson-fixes in 3 minutes and 31 seconds (queued for 21 seconds)
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