Expose custom Rust registry versions

This allows users to specify a custom registry src,
because currently every packager would need to create
an outdated Cargo.lock just to be compatible with the
probably outdated rustRegistry in nixpkgs.

Currently there is no easy way to convince cargo to
do that, so this makes that workaround unnecessary.

pkgs/build-support/rust/default.nix

@@ -1,4 +1,4 @@
-{ stdenv, cacert, git, rust, rustRegistry }:
+{ stdenv, callPackage, path, cacert, git, rust }:
 { name, depsSha256
 , src ? null
 , srcs ? null
@@ -8,9 +8,14 @@
 , cargoUpdateHook ? ""
 , cargoDepsHook ? ""
 , cargoBuildFlags ? []
+, registry ? null
 , ... } @ args:
 
 let
+  lib = stdenv.lib;
+  rustRegistry = callPackage (path + /pkgs/top-level/rust-packages.nix)
+    (lib.optionalAttrs (registry != null) { src = registry; });
+
   fetchDeps = import ./fetchcargo.nix {
     inherit stdenv cacert git rust rustRegistry;
   };