[OCI] revert pkgs.file

nix/default.nix

@@ -2,7 +2,7 @@
 { pkgs, stdenv, src ? ../., opam2nix, ocaml_version ? "ocaml-ng.ocamlPackages_4_05.ocaml", plugins ? { } }:
 
 let mk_buildInputs = { opamPackages ? [], nixPackages ? [] } :
-    [ pkgs.gnugrep pkgs.gnused  pkgs.autoconf pkgs.gnumake pkgs.gcc pkgs.ncurses pkgs.time pkgs.python3 pkgs.perl] ++ nixPackages ++ opam2nix.build {
+    [ pkgs.gnugrep pkgs.gnused  pkgs.autoconf pkgs.gnumake pkgs.gcc pkgs.ncurses pkgs.time pkgs.python3 pkgs.perl pkgs.file ] ++ nixPackages ++ opam2nix.build {
            specs = opam2nix.toSpecs ([ "ocamlfind" "zarith" "ocamlgraph" "yojson"
                 { name = "coq"; constraint = "=8.7.2"; }
                 ] ++ opamPackages ++