CI: use macos 11 instead of 10.15
CI: use macos 11 instead of 10.15

10.15 is deprecated, see
https://github.com/actions/virtual-environments/issues/5583
it causes random failures eg https://github.com/coq/coq/actions/runs/2712348619

coq/coq,Zimmi48/coq,SkySkimmer/coq,ejgallego/coq,SkySkimmer/coq,Zimmi48/coq,ejgallego/coq,Zimmi48/coq,coq/coq,SkySkimmer/coq,ejgallego/coq,SkySkimmer/coq,ejgallego/coq,coq/coq,coq/coq,Zimmi48/coq