buildRustPackage: fix deprecated use of registry.index config key (#22987)
buildRustPackage: fix deprecated use of registry.index config key (#22987)

See
https://github.com/rust-lang/cargo/commit/8214bb953dee7f529747ce69ff81fe294259a6a0
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
SymbiFlow/nixpkgs,SymbiFlow/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,SymbiFlow/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,SymbiFlow/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,SymbiFlow/nixpkgs,SymbiFlow/nixpkgs,SymbiFlow/nixpkgs,NixOS/nixpkgs,SymbiFlow/nixpkgs,SymbiFlow/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,SymbiFlow/nixpkgs,SymbiFlow/nixpkgs,SymbiFlow/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,SymbiFlow/nixpkgs