[ci] [appveyor] Disable validate target.
[ci] [appveyor] Disable validate target.

Not a big point on running the checker on Appveyor I think, this will
save a bunch of time and improve reliability.

ppedrot/coq,SkySkimmer/coq,Zimmi48/coq,coq/coq,maximedenes/coq,SkySkimmer/coq,SkySkimmer/coq,coq/coq,ppedrot/coq,Matafou/coq,maximedenes/coq,JasonGross/coq,maximedenes/coq,ppedrot/coq,JasonGross/coq,Zimmi48/coq,Matafou/coq,JasonGross/coq,Zimmi48/coq,maximedenes/coq,ejgallego/coq,silene/coq,ppedrot/coq,ejgallego/coq,gares/coq,SkySkimmer/coq,silene/coq,Zimmi48/coq,gares/coq,coq/coq,gares/coq,silene/coq,coq/coq,ejgallego/coq,maximedenes/coq,gares/coq,ejgallego/coq,gares/coq,JasonGross/coq,Matafou/coq,ppedrot/coq