Log Message:
rebar3: update to version 3.20.0

Changelog (taken from https://github.com/erlang/rebar3/releases/tag/3.20.0):

This release took a while to build; OTP-25.1 had incompatibilities with EUnit
that interrupted the CI process, and which are corrected in later releases.

New features:
    Add --eval option to shell
    Add support for dialyzer --incremental when available
    Vendor structure

This new vendor structure should be of no impact to anyone using Rebar3 as a
built artifact, but should make the life of corporate users and
package managers much easier by having pre-bundled in all dependencies and
allowing effective offline builds. It also simplifies our bootstrap code and
reduces security issues by not having to download a certificate bundle
without certificates, at least until our oldest supported version can use
Erlang's new built-in on-host certificate lookup mechanisms.

If you do rely on Rebar3 source for your tooling or any other mechanisms,
then you will want to cd apps/rebar before running other commands.

Other fixes:
    Warn of EUnit issues on OTP-25.1
    Respect HEX_MIRROR environment variable
    Use shell:start_interactive to start shell if available
    Do not send empty If-None-Match in tarball request
    Respect rebar_packages_cdn set in rebar.config
    Dereference symbolic links when copying files from ct data folder
    Reduce warnings around bootstrap and DAG operations