Take Nickel grammar directly from the flake

The tree-sitter Nickel grammar taken from Nixpkgs is too old and doesn't
match current queries, and the update process to upstream an update is
not obvious. In the meantime, we take the tree-sitter-nickel grammar
directly from the source repo - as a flake input - to ensure it's up to
date.

default.nix

@@ -5,6 +5,11 @@
 , rust-overlay
 , nix-filter
 , craneLib
+# tree-sitter-Nickel is packaged in Nixpkgs, but it's an older version at the
+# time of writing. Since updating it seems non trivial, and we need Topiary to
+# be compatible with Nickel urgently (it is currently blocking for the CI), we
+# use the tree-sitter-nickel flake directly.
+, tree-sitter-nickel
 }:
 let
   wasmRustVersion = "1.77.2";
@@ -123,7 +128,7 @@ in
       bash="${pkgs.tree-sitter-grammars.tree-sitter-bash}/parser" \
       css="${pkgs.tree-sitter-grammars.tree-sitter-css}/parser" \
       json="${pkgs.tree-sitter-grammars.tree-sitter-json}/parser" \
-      nickel="${pkgs.tree-sitter-grammars.tree-sitter-nickel}/parser" \
+      nickel="${tree-sitter-nickel}/parser" \
       ocaml="${pkgs.tree-sitter-grammars.tree-sitter-ocaml}/parser" \
       ocaml_interface="${pkgs.tree-sitter-grammars.tree-sitter-ocaml-interface}/parser" \
       rust="${pkgs.tree-sitter-grammars.tree-sitter-rust}/parser" \

flake.nix

@@ -27,6 +27,12 @@
 
     flake-utils.url = "github:numtide/flake-utils";
     nix-filter.url = "github:numtide/nix-filter";
+
+    tree-sitter-nickel-input = {
+      url = "github:nickel-lang/tree-sitter-nickel";
+      inputs.nixpkgs.follows = "nixpkgs";
+      inputs.flake-utils.follows = "flake-utils";
+    };
   };
 
   outputs = inputs:
@@ -55,7 +61,11 @@
 
         craneLib = crane.mkLib pkgs;
 
-        topiaryPkgs = pkgs.callPackage ./default.nix { inherit advisory-db crane rust-overlay nix-filter craneLib; };
+        tree-sitter-nickel = tree-sitter-nickel-input.packages.${system}.default;
+
+        topiaryPkgs = pkgs.callPackage ./default.nix {
+          inherit advisory-db crane rust-overlay nix-filter craneLib tree-sitter-nickel;
+        };
         binPkgs = pkgs.callPackage ./bin/default.nix { };
       in
       {