With the hex plugin download it would cause rebar3 tests to fail on any
VM that couldn't load the pre-compiled plugin modules. This just hides
the config and plugin cache from rebar3 when running under this test
script.
It appears that whatever previous prevented me from using the rebar3
port_compiler is no longer an issue. This change allows rebar2 and
rebar3 compilation by relying on port_compiler for rebar3 and
reverting to standard rebar2 otherwise.
Paul J. Davis