ci: Condition Windows containers on Windows rules

The Windows container is not necessary for changes that do not affect
Windows. As only a small number of Mesa drivers run on Windows,
this means that most hardware driver-specific changes can skip the
Windows container build.

Last night there was an infrastructure issue external to us causing the
Windows container builds to timeout. While this would not have fixed
that or helped with common code MRs, at least it would've let me merge
Panfrost code in the mean time ;-)

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
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