Add some notes to flake.nix and nix/prefetcher.nix

Notes are to describe `pkgs.buildGoModule` mapping in Nixpkgs.

flake.nix

@@ -33,6 +33,8 @@
           version = substring 0 8 self.rev or "dirty";
           vendorSha256 = readFile ./nix/vendor-sha256;
           buildGoModule = pkgs.buildGoModule;
+            # As of writing, `pkgs.buildGoModule` is aliased to
+            # `pkgs.buildGo121Module` in Nixpkgs.
         };
         # Build packwiz by default when no package name is specified
         default = packwiz;