logging: handle build log lines in simple logger

The raw stderr output isn't logged anymore so the build logs need to be
printed by the default logger in order for the old commands like
nix-build to still show build output.

src/libutil/logging.cc

@@ -63,6 +63,16 @@ public:
         writeToStderr(prefix + filterANSIEscapes(fs.s, !tty) + "\n");
     }
 
+    void result(ActivityId act, ResultType type, const std::vector<Field> & fields) override
+    {
+        if (type == resBuildLogLine || type == resPostBuildLogLine) {
+            assert(0 < fields.size());
+            assert(fields[0].type == Logger::Field::tString);
+            auto lastLine = fields[0].s;
+            log(lvlInfo, lastLine);
+        }
+    }
+
     void startActivity(ActivityId act, Verbosity lvl, ActivityType type,
         const std::string & s, const Fields & fields, ActivityId parent)
         override