chore: add coq_8_16 to nix flake

ps-id: 54dbbeb5-ebea-4236-b220-2e0a573ac9cd
Signed-off-by: Ali Caglayan <alizter@gmail.com>

Author: Alizter

flake.nix

@@ -46,6 +46,7 @@
             # dev tools
             ocamlformat_0_21_0
             opam
+            coq_8_16
             nodejs-slim
             pkg-config
           ] ++ (builtins.map (s: builtins.getAttr s scope)