On Windows, tmp files cannot be deleted without closing it first.

core/src/main/java/hudson/util/AtomicFileWriter.java

@@ -117,6 +117,7 @@ public class AtomicFileWriter extends Writer {
     @Override
     protected void finalize() throws Throwable {
         // one way or the other, temporary file should be deleted.
+        close();
         tmpFile.delete();
     }