std: Swap arguments in Thread.spawn

Beside the new order being consistent with the ThreadPool API and making
more sense, this shuffling allows to write the context argument type in
terms of the startFn arguments, reducing the use of anytype (eg. less
explicit casts when using comptime_int parameters, yay).

Sorry for the breakage.

Closes #8082

src/ThreadPool.zig

@@ -74,7 +74,7 @@ pub fn init(self: *ThreadPool, allocator: *std.mem.Allocator) !void {
         try worker.idle_node.data.init();
         errdefer worker.idle_node.data.deinit();
 
-        worker.thread = try std.Thread.spawn(worker, Worker.run);
+        worker.thread = try std.Thread.spawn(Worker.run, worker);
     }
 }