Improve overrides.nix

[deemp]

• [x] According to https://github.com/nix-community/nix-vscode-extensions/issues/31#issuecomment-1951128781, we may not need to set sourceRoot = "extension" when https://github.com/NixOS/nixpkgs/pull/289721 is merged.
• [ ] Adopt workarounds from nixpkgs
  • https://github.com/NixOS/nixpkgs/issues/270423#issuecomment-1902482401
• [x] Simplify and publish overrides.nix to a separate branch without all cache files so that other solutions like nix4vscode can make use of overrides.

[FraGag]

https://github.com/NixOS/nixpkgs/pull/289721 was just merged into master. Once it gets merged into nixpkgs-unstable and nixos-unstable, then we'll be able to remove sourceRoot = "extension" in overrides.nix.

I advise waiting at least until it's in nixos-unstable, in case someone uses nix-vscode-extensions in a flake that changes its nixpkgs input to follow the nixos-unstable branch. If we want to be extra cautious, we could also wait for https://github.com/NixOS/nixpkgs/pull/303445, which backports the change to the release-23.11 branch.