Fix wrong period for creating nested ctx in TreeTransforms.
Fix wrong period for creating nested ctx in TreeTransforms.

`owner` for nestedContext in `TreeTransforms` was assigned
using initial period of whole `TreeTransforms` block.
That is incorrect as denotation for this owner could have been
create by some phase in this block.

sjrd/dotty,lampepfl/dotty,vsalvis/dotty,sjrd/dotty,dotty-staging/dotty,folone/dotty,sjrd/dotty,vsalvis/dotty,som-snytt/dotty,lampepfl/dotty,lampepfl/dotty,svalaskevicius/dotty,sjrd/dotty,auvik/dotty,yusuke2255/dotty,auvik/dotty,AlexSikia/dotty,som-snytt/dotty,reactormonk/dotty,angi2/dotty,som-snytt/dotty,lampepfl/dotty,spetz911/dotty,yusuke2255/dotty,lampepfl/dotty,AlexSikia/dotty,som-snytt/dotty,sjrd/dotty,dotty-staging/dotty,densh/dotty,som-snytt/dotty,densh/dotty,angi2/dotty,VladimirNik/dotty,folone/dotty,dotty-staging/dotty,reactormonk/dotty,dotty-staging/dotty,spetz911/dotty,svalaskevicius/dotty,dotty-staging/dotty,VladimirNik/dotty