Fix progress bar flicker with -L

This was caused by -L calling setLogFormat() again, which caused the
creation of a new progress bar without destroying the old one. So we
had two progress bars clobbering each other.

We should change 'logger' to be a smart pointer, but I'll do that in a
future PR.

Fixes #6931.

src/libutil/logging.hh

@@ -111,6 +111,9 @@ public:
 
     virtual std::optional<char> ask(std::string_view s)
     { return {}; }
+
+    virtual void setPrintBuildLogs(bool printBuildLogs)
+    { }
 };
 
 ActivityId getCurActivity();