Update pinned nixpkgs to use Dune 1.6.
Update pinned nixpkgs to use Dune 1.6.

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