feat(tvix): Thread a log sink through calls to buildDerivation

Similarly to how we did for buildPaths, add a std::ostream& log_sink
parameter to the build_derivation method on Store, and pass it std::cerr
when called at the top level by nix commands - most notably, the
build-remote hook binary, so that we get build logs when using tvix as a
remote builder.

Change-Id: I0f8f729ba8429d4838a0a135a5c2ac1e1a95d575
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2176
Tested-by: BuildkiteCI
Reviewed-by: andi <andi@notmuch.email>
Reviewed-by: kanepyork <rikingcoding@gmail.com>

third_party/nix/src/libstore/binary-cache-store.hh

@@ -93,7 +93,8 @@ class BinaryCacheStore : public Store {
 
   void narFromPath(const Path& path, Sink& sink) override;
 
-  BuildResult buildDerivation(const Path& drvPath, const BasicDerivation& drv,
+  BuildResult buildDerivation(std::ostream& /*log_sink*/, const Path& drvPath,
+                              const BasicDerivation& drv,
                               BuildMode buildMode) override {
     unsupported("buildDerivation");
   }