rls: update preBuild for changes in buildRustPackage
rls: update preBuild for changes in buildRustPackage

preBuild is now run before changing to buildAndTestSubdir, so use full
path to tests/client.rs in preBuild.

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