fix: collection of cargo.nix dirs to be unique and not error when root

Cargo.nix hasn't been set

Author: Pacman99

nix/public-flake-module.nix

@@ -334,10 +334,10 @@ ourArgs:
           in builtins.dirOf (builtins.unsafeDiscardStringContext
             (lib.concatStringsSep "/" inRepoFileSplit));
 
-          cargoNixDirs = lib.map getCargoNixDir
-            ([ cfg.cargoNix.path ] ++
-            (lib.mapAttrsToList (_: v: v.cargoNix.path) cfg.extraBuilds));
-              
+          cargoNixDirs = lib.unique (lib.map getCargoNixDir
+            ((lib.optionals (cfg.cargoNix ? path) [ cfg.cargoNix.path ]) ++
+            (lib.mapAttrsToList (_: v: v.cargoNix.path) cfg.extraBuilds)));
+
         in {
           options.crate2nix = lib.mkOption {
             type = types.submoduleWith {