Improve overrides.nix

[deemp]

• According to Multiple directories #31 (comment), we may not need to set sourceRoot = "extension" when vscode-utils: Set the sourceRoot attribute on vscode extensions NixOS/nixpkgs#289721 is merged.
• Adopt workarounds from nixpkgs
  • Package request: vscode extension ms-dotnettools.csdevkit NixOS/nixpkgs#270423 (comment)
• 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]

NixOS/nixpkgs#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 NixOS/nixpkgs#303445, which backports the change to the release-23.11 branch.