fix: respect fetchurl passed via top-level Cargo.nix

This seems to be an overlook.

`fetchurl` for registries is using the one from function argument,
and falls back to `pkgs.fetchurl`.
This PR makes fetches for `.crate` tarballs aligned.

Author: weihanglo

crate2nix/templates/nix/crate2nix/default.nix

@@ -417,7 +417,7 @@ rec {
             crateConfig
             // {
               src =
-                crateConfig.src or (pkgs.fetchurl rec {
+                crateConfig.src or (fetchurl rec {
                   name = "${crateConfig.crateName}-${crateConfig.version}.tar.gz";
                   # https://www.pietroalbini.org/blog/downloading-crates-io/
                   # Not rate-limited, CDN URL.