RunPager: Stop the progress bar

In particular, the progress bar was interfering with 'less' rendering
in '--help' (e.g. run 'nix --help' and hit '/' to search).

src/libmain/shared.cc

@@ -4,6 +4,7 @@
 #include "gc-store.hh"
 #include "util.hh"
 #include "loggers.hh"
+#include "progress-bar.hh"
 
 #include <algorithm>
 #include <cctype>
@@ -422,6 +423,8 @@ RunPager::RunPager()
     if (!pager) pager = getenv("PAGER");
     if (pager && ((std::string) pager == "" || (std::string) pager == "cat")) return;
 
+    stopProgressBar();
+
     Pipe toPager;
     toPager.create();