Windows: enable MNX module only on CI

[RomanPudashkin]

As it requires xxd to be installed on Windows. And we don't want to make the build process even more difficult for new developers. @rpatters1 FYI

[rpatters1]

Is it possible to limit this just to the Win dev build? There is no reason not to build it on Linux or Mac devs, since those come with xxd installed. And the more computers running the build, the more likely we are to shake out bugs. (Heck, the Win dev build already shook out an issue.)

[rpatters1]

Also, I will add here (just to document it) that to intall xxd on Windows requires

choco install xxd -y

I believe the standard Windows dev setup already suggests using choco.

[cbjeukendrup]

Or can we do it based on the availability of xxd? I.e. look for xxd; if not found, print warning and skip mnx module

[rpatters1]

Thank you so much. The minute #embed is available on our compilers, I will get rid of xxd. I don't love it.