Add escape hatch MSBUILDCOPYWITHOUTDELETE (#9257)

This allows an opt-in workaround for #9250 that affected deployment
processes can use, mitigating the risk of entirely reverting #8685.

src/Framework/Traits.cs

@@ -188,6 +188,11 @@ internal class EscapeHatches
         /// </summary>
         public readonly bool AlwaysDoImmutableFilesUpToDateCheck = Environment.GetEnvironmentVariable("MSBUILDDONOTCACHEMODIFICATIONTIME") == "1";
 
+        /// <summary>
+        /// When copying over an existing file, copy directly into the existing file rather than deleting and recreating.
+        /// </summary>
+        public readonly bool CopyWithoutDelete = Environment.GetEnvironmentVariable("MSBUILDCOPYWITHOUTDELETE") == "1";
+
         /// <summary>
         /// Emit events for project imports.
         /// </summary>

src/Tasks/Copy.cs

@@ -285,7 +285,10 @@ private void LogAlwaysRetryDiagnosticFromResources(string messageResourceName, p
                 MakeFileWriteable(destinationFileState, true);
             }
 
-            if (ChangeWaves.AreFeaturesEnabled(ChangeWaves.Wave17_8) && destinationFileState.FileExists && !destinationFileState.IsReadOnly)
+            if (ChangeWaves.AreFeaturesEnabled(ChangeWaves.Wave17_8) &&
+                Traits.Instance.EscapeHatches.CopyWithoutDelete != true &&
+                destinationFileState.FileExists &&
+                !destinationFileState.IsReadOnly)
             {
                 FileUtilities.DeleteNoThrow(destinationFileState.Name);
             }