nim: use newer bootstrap repository for aarch64-darwin support
nim: use newer bootstrap repository for aarch64-darwin support

The old "csources" repo has been archived a while ago and a newer
csources_v1 repo has been published. The main advantage is that we now
have support for newer platorms (such as Apple M1's) and newer nim
versions in the bootstrap.

The new repository doesn't contain release tags thus I pinned it to the
latest revision that has been published.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs