ci: make the build 'Setup Ninja' job windows-only

[mizvekov]

Ninja comes pre-installed in every other image except Windows.

[cppalliance-bot]

An automated preview of the documentation is available at https://947.mrdocs.prtest2.cppalliance.org/index.html

[alandefreitas]

Maybe we can just leave ninja there for all platforms? It's what we've been doing in the bootstrap script anyway.