Make the logger customisable

Add a new `--log-format` cli argument to change the format of the logs.
The possible values are
- raw (the default one for old-style commands)
- bar (the default one for new-style commands)
- bar-with-logs (equivalent to `--print-build-logs`)
- internal-json (the internal machine-readable json format)

src/nix-env/nix-env.cc

@@ -1446,6 +1446,8 @@ static int _main(int argc, char * * argv)
 
         globals.state->printStats();
 
+        logger->stop();
+
         return 0;
     }
 }