buildRustPackage: fix deprecated use of registry.index config key (#2…

…2987)

See
rust-lang/cargo@8214bb9
for the cargo commit which deprecated the registry.index key, and
implements this as a replacement. This gets rid of the error message

warning: custom registry support via the `registry.index` configuration is being removed, this functionality will not work in the future

pkgs/build-support/rust/fetch-cargo-deps

@@ -11,8 +11,11 @@ fetchCargoDeps() {
     echo "Using rust registry from $rustRegistry"
 
     cat <<EOF > $out/config
-[registry]
-index = "file://$rustRegistry"
+[source.nix-store-rust-registry]
+registry = "file://$rustRegistry"
+
+[source.crates-io]
+replace-with = "nix-store-rust-registry"
 EOF
 
     export CARGO_HOME=$out