flake.nix: add rust-src to devShell

Dependencies

• [2] ZWPPIJXF flake.nix: use Mozilla overlay for Rust
• [3] G734WNM6 flake.nix: use crate2nix
• [*] SXEYMYF7 Fixing the bad changes in history (unfortunately, by rebooting).

Change contents

• replacement in flake.nix at line 74
  [2.809]→[2.809:838](∅→∅)
  − rustChannel.rust
  [2.809]

  [3.246506]
  + (rustChannel.rust.override { extensions = [ "rust-src" ]; })