Fix warning. use compiletime.uninitialized

[xuwei-k]

[warn] -- Warning: /home/runner/work/io/io/io/src/main/scala/sbt/internal/io/DeferredWriter.scala:19:34 
[warn] 19 |  private var delegate0: Writer = _
[warn]    |                                  ^
[warn]    |`= _` has been deprecated; use `= uninitialized` instead.
[warn]    |`uninitialized` can be imported with `scala.compiletime.uninitialized`.
[warn]    |This construct can be rewritten automatically under -rewrite -source 3.4-migration.

[eed3si9n]

Thanks!