[ci] Add OCP-indent

nix/ocp-indent.nix

+{ lib
+, fetchzip
+, buildDunePackage
+, cmdliner
+}:
+
+buildDunePackage rec {
+  version = "1.7.0";
+  pname = "ocp-indent";
+
+  src = fetchzip {
+    url = "https://github.com/OCamlPro/ocp-indent/archive/${version}.tar.gz";
+    sha256 = "006x3fsd61vxnxj4chlakyk3b2s10pb0bdl46g0ghf3j8h33x7hc";
+  };
+
+  minimumOCamlVersion = "4.02";
+  useDune2 = true;
+
+  buildInputs = [ cmdliner ];
+
+  meta = with lib; {
+    homepage = http://typerex.ocamlpro.com/ocp-indent.html;
+    description = "A customizable tool to indent OCaml code";
+    license = licenses.gpl3;
+    maintainers = [ maintainers.jirkamarsik ];
+  };
+}

nix/pkgs.nix

@@ -4,6 +4,7 @@ let
     # External Packages
     alt-ergo = oself.callPackage ./alt-ergo.nix {};
     camlzip = oself.callPackage ./camlzip.nix {};
+    ocp-indent = oself.callPackage ./ocp-indent.nix {};
     psmt2-frontend = oself.callPackage ./psmt2-frontend.nix {};
     why3 = oself.callPackage ./why3.nix {};
     # Builds