Customize Forest data directory via env variable

[LesnyRumcajs]

Summary

Forest data directory is customizable exclusively with a configuration file. We could make it easier by allowing to pass an environmental variable like FOREST_PATH (in Lotus, there's a LOTUS_PATH so it'd make easier for folks switching between implementations).

Precedence:

1. Env variable
2. Config
3. Defaults

Other binaries might also need to be updated, e.g., forest-wallet.

Completion Criteria

• Introduce FOREST_PATH environmental variable and ensure it's consistent with other forest* binaries
• Look into current configuration files in Forest test scripts - perhaps some logic could be simplified thanks to this.
• Add entry in docs

Additional Links & Resources

[hanabi1224]

I think it'd also be helpful to log the actual data dir on startup.