Nix: Fix clangd not finding system libs

The clangd from clang-tools is patched to find the correct libraries

The ms-vscode.cpptools extension for Visual Studio Code is somehow broken in that it uses one CPU thread at 100% all the time then crashes and restarts every 10min or so. The extension still partly works which was good enough to work.

The alternative llvm-vs-code-extensions.vscode-clangd extension relies on the clangd LSP server which was failing to find system libs on my system (e.g. <string.h>).

With that fixed I can finally ditch the cpptools extension.