Clear _build/bootstrap on each bootstrap run
Clear _build/bootstrap on each bootstrap run

This should avoid common problems when upgrading across versions where
old code kind of messes up a lot of things and clearing _build keeps
being required.

rebar/rebar3,erlang/rebar3,vladdu/rebar3,tsloughter/rebar3,rebar/rebar3,ferd/rebar3,saleyn/rebar3,djnym/rebar3,lrascao/rebar3