Use Builder in ImmutableDictionary.SetItems extension (#12402)

### Context

Small change to avoid allocating a generated enumerable. Using a
`Builder` here is otherwise identical in perf to `SetItems`, as both hit
the same paths to allocate a new instance + modified subtree nodes.
~34MB diff below.

Author: ccastanedaucf

src/Framework/ImmutableDictionaryExtensions.cs

@@ -16,6 +16,7 @@ internal static class ImmutableDictionaryExtensions
         public static readonly ImmutableDictionary<string, string> EmptyMetadata =
             ImmutableDictionary<string, string>.Empty.WithComparers(MSBuildNameIgnoreCaseComparer.Default);
 
+#if !TASKHOST
         /// <summary>
         /// Sets the given items while running a validation function on each key.
         /// </summary>
@@ -29,23 +30,18 @@ public static ImmutableDictionary<string, string> SetItems(
             IEnumerable<KeyValuePair<string, string>> items,
             Action<string> verifyThrowKey)
         {
-            return dictionary.SetItems(ValidateItems(items, verifyThrowKey));
+            ImmutableDictionary<string, string>.Builder builder = dictionary.ToBuilder();
 
-            // PERF: This extra allocation is still faster than enumerating ImmutableDictionary after modification.
-            static IEnumerable<KeyValuePair<string, string>> ValidateItems(
-                IEnumerable<KeyValuePair<string, string>> items,
-                Action<string> verifyThrowKey)
+            foreach (KeyValuePair<string, string> item in items)
             {
-                foreach (KeyValuePair<string, string> item in items)
-                {
-                    verifyThrowKey(item.Key);
+                verifyThrowKey(item.Key);
 
-                    // Set null as empty string to match behavior with ProjectMetadataInstance.
-                    yield return item.Value == null
-                        ? new KeyValuePair<string, string>(item.Key, string.Empty)
-                        : item;
-                }
+                // Set null as empty string to match behavior with ProjectMetadataInstance.
+                builder[item.Key] = item.Value ?? string.Empty;
             }
+
+            return builder.ToImmutable();
         }
+#endif
     }
 }