Upgrade reference compiler to latest nightly
Upgrade reference compiler to latest nightly

In particular, this contains #8638 which avoids getting some confusing
error messages.

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