Uses non-standard logs directory on macOS

[ErikBjare]

aw-server-rust stored logs in to the ~/Library/Cache/activitywatch/log, but on macOS logs should go in ~/Library/Logs.

This is different from other ActivityWatch modules, and notably, aw-qt opens ~/Library/Logs/activitywatch when "Open log folder" is clicked.

Edit: I notice that appdirs has a get_log_dir that behaves correctly, we're just not using it.