Help grpc find libstdc++.so.8

Prebuilt grpc stuff is annoying:

    vendor/python/3.9/packages/lib/python3.9/site-packages/grpc/_compression.py:20: in <module>
        from grpc._cython import cygrpc
    E   ImportError: libstdc++.so.6: cannot open shared object file: No such file or directory

Author: lloeki

shell.nix

@@ -39,5 +39,7 @@ in pinned.stdenv.mkDerivation {
     unset SOURCE_DATE_EPOCH
     export PATH="$PIP_PREFIX/bin:$PATH"
 
+    # hack: can't find libstdc++.so.8 otherwise
+    export LD_LIBRARY_PATH="${pinned.stdenv.cc.cc.lib}/lib"
   '';
 }