chore(tvix): Thread a std::ostream through Store::buildPaths

This part of the store API needs to carry a handle to the log sink
from now on, so that it can be passed in as appropriate from the gRPC
handlers.

In all places where there is no such handler available at the moment,
the discarding log sink has been inserted. This can be used as a
convenient grep target in the future.

Change-Id: I26628e30b4c6437dccdf8f722ca2e8ed827dfc19
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1797
Tested-by: BuildkiteCI
Reviewed-by: kanepyork <rikingcoding@gmail.com>
Reviewed-by: glittershark <grfn@gws.fyi>

third_party/nix/src/libstore/store-api.cc

@@ -714,7 +714,8 @@ const Store::Stats& Store::getStats() {
   return stats;
 }
 
-absl::Status Store::buildPaths(const PathSet& paths, BuildMode) {
+absl::Status Store::buildPaths(std::ostream& /* log_sink */,
+                               const PathSet& paths, BuildMode) {
   for (auto& path : paths) {
     if (isDerivation(path)) {
       return absl::Status(absl::StatusCode::kUnimplemented,