Fix repeated bootstrap runs
Fix repeated bootstrap runs

Updating the bootstrap script in #1689 made it so symlinks that already
exist return `exist` rather than ok, in line with the regular code.
However the bootstrap module wouldn't handle this case.

This fixes the problem by ensuring that `exists` is as valid as `ok`.

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