feat(overrides.nix): remove sourceRoot

[DontEatOreo]

Just making this PR ahead of time. I think this should be merged when (https://github.com/NixOS/nixpkgs/pull/289721) reaches nixos-unstable. We can keep track here at https://nixpk.gs/pr-tracker.html?pr=289721

• Reference: https://github.com/nix-community/nix-vscode-extensions/issues/55
• Since https://github.com/NixOS/nixpkgs/pull/289721 and https://github.com/NixOS/nixpkgs/pull/303445 got merged we can safely remove sourceRoot

[deemp]

@DontEatOreo, please ping me when it reaches nixos-unstable.

[DontEatOreo]

@deemp It has reached nixos-unstable