Integrate Saturn L2 v0.0.8 and configure ROOT_DIR

[juliangruber]

Once a new release is cut https://github.com/filecoin-project/saturn-l2

[bajtos]

As part of this work, I'd like us to configure the new ROOT_DIR env var accepted by Saturn L2. This variable controls the location where the L2 node keeps the cache.

We must choose the right location for this cache:

• %LOCALAPPDATA% on Windows
• XDG_CACHE_HOME on Linux
• ~/Library/Caches/ on macOS

See ipfs/ipfs-desktop#1656, electron/electron#8124, electron/electron#1404

[bajtos]

In particular, we cannot use Electron's APIs for choosing the cache dir location.

[bajtos]

See https://github.com/filecoin-saturn/L2-node/releases/tag/v0.0.7

[bajtos]

In particular, we cannot use Electron's APIs for choosing the cache dir location.

FWIW, here is an Electron PR trying to simplify access to the platform-specific cache/local-app-data directory: electron/electron#34337