Changes

testenv: podman: don't install GUI packages

Install erlang-nox and use the pre-built rebar3 as linked from
rebar3.org, instead of using the Debian package to avoid pulling in
~600 MB of GUI dependencies.

Related: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1083096
Change-Id: I27f58d1f102cf85ff7ff301c52696b65150e2898