[default.nix] Unpin nixpkgs
[default.nix] Unpin nixpkgs

Sphinx dependencies are now available in unstable channel

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