nix run: Flush the progress bar before starting the command

src/nix/edit.cc

@@ -2,6 +2,7 @@
 #include "shared.hh"
 #include "eval.hh"
 #include "attr-path.hh"
+#include "progress-bar.hh"
 
 #include <unistd.h>
 
@@ -65,6 +66,8 @@ struct CmdEdit : InstallablesCommand
 
             args.push_back(filename);
 
+            stopProgressBar();
+
             execvp(editor.c_str(), stringsToCharPtrs(args).data());
 
             throw SysError("cannot run editor '%s'", editor);