[perl #132275] a Promise using a `temp`ed variable gets untemped version

[Zoffix Znet via RT]

Notabug: https://irclog.perlgeek.de/perl6-dev/2017-10-12#i_15293653

12:50   jnthn   start blocks don't make a complete clone of the dynamic scope, 
so they'll see any mutations in it
12:50           They just have a reference to the context they were invoked in