Build.scala: Fix dottydoc bootstrap compilation
Build.scala: Fix dottydoc bootstrap compilation

Previously, nothing was compiled because the directory
doc-tool-bootstrapped does not exist.

som-snytt/dotty,lampepfl/dotty,lampepfl/dotty,sjrd/dotty,lampepfl/dotty,dotty-staging/dotty,som-snytt/dotty,lampepfl/dotty,som-snytt/dotty,som-snytt/dotty,sjrd/dotty,som-snytt/dotty,sjrd/dotty,dotty-staging/dotty,dotty-staging/dotty,sjrd/dotty,dotty-staging/dotty,dotty-staging/dotty,lampepfl/dotty,sjrd/dotty