[ci] [appveyor] Enable cache for builds.
[ci] [appveyor] Enable cache for builds.

We cache CYGWIN and OPAM. Current cygwin install is not deterministic
but should do the job in the scope of this job.

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