[ci] tries to fix coq version

nix/pkgs.nix

@@ -8,7 +8,9 @@ let
     odoc = oself.callPackage ./odoc.nix {};
     odoc-parser = oself.callPackage ./odoc-parser.nix {};
     ppxlib = oself.callPackage ./ppxlib.nix {};
-    why3 = oself.callPackage ./why3.nix {};
+    why3 = oself.callPackage ./why3.nix {
+      coqPackages = pkgs.mkCoqPackages (pkgs.coq.override { version = "8.18";});
+    };
 
     # Helpers
     mk_tests = oself.callPackage ./mk_tests.nix {};