fix(tvix): Remove more superfluous log output

- A couple of log messages had double-newlines still
- Remove the [ drvPath ] bit from the "building path" output, since the
  derivation path is already present in the message itself.

Change-Id: Ib4b514a18fcb63c7191f4d7cf0a9b857aaa3f3fa
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2171
Tested-by: BuildkiteCI
Reviewed-by: kanepyork <rikingcoding@gmail.com>

third_party/nix/src/libstore/build.cc

@@ -1472,7 +1472,7 @@ void DerivationGoal::tryToBuild() {
     if (hook) {
       msg += fmt(" on '%s'", machineName);
     }
-    log_sink() << absl::StrCat(msg, "[", drvPath, "]\n") << std::endl;
+    log_sink() << msg << std::endl;
     mcRunningBuilds =
         std::make_unique<MaintainCount<uint64_t>>(worker.runningBuilds);
   };

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

@@ -364,8 +364,7 @@ absl::Status RpcStore::buildPaths(std::ostream& log_sink, const PathSet& paths,
       // TODO(tazjin): Include .path()?
       log_sink << event.build_log().line();
     } else {
-      log_sink << std::endl
-               << "Building path: " << event.building_path().path()
+      log_sink << "Building path: " << event.building_path().path()
                << std::endl;
     }