Avoid overly-specific cast in bulk metadata copy (#8646)

The return value of `ITaskItem.CloneCustomMetadata` is an `IDictionary`,
which is generally (in modern MSBuild) backed by a
`Dictionary<string, string>`, but can be (when given an item from a
net35 taskhost) a `Hashtable`. In the latter situation, casting entries
to `KeyValuePair<,>` fails, because they conform only to
`DictionaryEntry`.

Use that less-well-typed approach--the casts were present in the pre-
bulk-edit version of the code. Fixes #8645.

Author: rainersigwald

eng/Versions.props

@@ -2,7 +2,7 @@
 <!-- Copyright (c) .NET Foundation and contributors. All rights reserved. Licensed under the MIT license. See License.txt in the project root for full license information. -->
 <Project>
   <PropertyGroup>
-    <VersionPrefix>17.6.0</VersionPrefix><DotNetFinalVersionKind>release</DotNetFinalVersionKind>
+    <VersionPrefix>17.6.1</VersionPrefix><DotNetFinalVersionKind>release</DotNetFinalVersionKind>
     <PackageValidationBaselineVersion>17.5.0</PackageValidationBaselineVersion>
     <AssemblyVersion>15.1.0.0</AssemblyVersion>
     <PreReleaseVersionLabel>preview</PreReleaseVersionLabel>

src/Build/BackEnd/TaskExecutionHost/TaskExecutionHost.cs

@@ -1391,8 +1391,8 @@ private void GatherTaskItemOutputs(bool outputTargetIsItem, string outputTargetN
                                     newItem = new ProjectItemInstance(_projectInstance, outputTargetName, EscapingUtilities.Escape(output.ItemSpec), parameterLocationEscaped);
 
                                     newItem.SetMetadataOnTaskOutput(output.CloneCustomMetadata()
-                                        .Cast<KeyValuePair<string, string>>()
-                                        .Select(x => new KeyValuePair<string, string>(x.Key, EscapingUtilities.Escape(x.Value))));
+                                        .Cast<DictionaryEntry>()
+                                        .Select(x => new KeyValuePair<string, string>((string)x.Key, EscapingUtilities.Escape((string)x.Value))));
                                 }
                             }