std.process.cleanExit: lock stderr before exiting

This makes it so that any other threads which are writing to stderr have
a chance to finish before the process terminates. It also clears the
terminal in case any progress has been written to stderr, while still
accomplishing the goal of not waiting until the update thread exits.

lib/std/process.zig

@@ -1760,6 +1760,7 @@ pub fn cleanExit() void {
     if (builtin.mode == .Debug) {
         return;
     } else {
+        std.debug.lockStdErr();
         exit(0);
     }
 }