Fix ObjectDisposedException in BuildsWhileBuildIsRunningOnServer test

src/MSBuild.UnitTests/MSBuildServer_Tests.cs

@@ -190,8 +190,11 @@ public void BuildsWhileBuildIsRunningOnServer()
             _env.WithTransientProcess(pidOfServerProcess);
 
             string? dir = Path.GetDirectoryName(markerFile.Path);
-            using var watcher = new System.IO.FileSystemWatcher(dir!);
+            // mre must be declared before watcher so that it is disposed after watcher.
+            // Reversing this order would allow late FileSystemWatcher callbacks to call
+            // mre.Set() on a disposed ManualResetEvent, causing an ObjectDisposedException.
             using ManualResetEvent mre = new ManualResetEvent(false);
+            using var watcher = new System.IO.FileSystemWatcher(dir!);
             watcher.Created += (o, e) =>
             {
                 _output.WriteLine($"The marker file {markerFile.Path} was created. The build task has been started.");