This adds changes to the rebar3 expression that patch rebar3 to force it
to be hermetic. Now, by default, rebar3 literally can't download
anything. A 'rebar3-open' expression was added for those folks whe want
the normal rebar3.
To successfully build rebar packages, it needs to be provided with
rebar3 plugins used to build it. This change passes them to env
variable. From there rebar3-nix-bootstrap takes them and symlinks into
_build/default/plugins.