Improve compatibility with ExAtomVM environment options

[UncleGrumpy]

To improve the compatibility with the ExAtomVM mix plugin, tasks should fall back to using the environment option overrides used by ExAtomVM. The rebar3 plugin variants should be retained and take precedence if they are set, giving users the option to use whichever suits their needs.

This should not change the option selection hierarchy:

1. Command line
2. rebar.config plugin configuration
3. REBAR_PLUGIN environment setting
4. ExAtomVM environment setting
5. Task fallback (if one exists)