set `run / fork := true`

[xuwei-k]

• https://github.com/sbt/sbt/issues/7408
• https://github.com/scalameta/mdoc/blob/d7b59fbd331786dfda17427a43a37520ebfedfdb/mdoc-sbt/src/main/scala/mdoc/MdocPlugin.scala#L85-L93