Remove unnecessary lock in TerminalLogger.UpdateNodeStatus (#10045)

This is a minor perf improvement.

In particular when using the project cache with a high cache rate (thus spamming ProjectFinished events), this lock is taking ~6% of the CPU for the process due to contention. That's a lower bound though because I'm seeing other methods which do take the lock also suffer from high contention (eg `ThreadProc` also takes ~6% CPU), so removing the lock from `UpdateNodeStatus` is likely to reduce contention there as well.

This lock isn't necessary since this operation is a simple object replacement in an array. Other similar operations also don't take a lock, (See the write in `ProjectStarted` and the read in `MessageRaised`) which proves that it's not required.

src/MSBuild/TerminalLogger/TerminalLogger.cs

@@ -91,6 +91,10 @@ public ProjectContext(BuildEventContext context)
     /// <summary>
     /// Tracks the work currently being done by build nodes. Null means the node is not doing any work worth reporting.
     /// </summary>
+    /// <remarks>
+    /// There is no locking around access to this data structure despite it being accessed concurrently by multiple threads.
+    /// However, reads and writes to locations in an array is atomic, so locking is not required.
+    /// </remarks>
     private NodeStatus?[] _nodes = Array.Empty<NodeStatus>();
 
     /// <summary>
@@ -701,11 +705,8 @@ private void TargetStarted(object sender, TargetStartedEventArgs e)
 
     private void UpdateNodeStatus(BuildEventContext buildEventContext, NodeStatus? nodeStatus)
     {
-        lock (_lock)
-        {
-            int nodeIndex = NodeIndexForContext(buildEventContext);
-            _nodes[nodeIndex] = nodeStatus;
-        }
+        int nodeIndex = NodeIndexForContext(buildEventContext);
+        _nodes[nodeIndex] = nodeStatus;
     }
 
     /// <summary>