Allow VARIABLE_DATA to be set in the profile instead of hardcoded

Fixes #311 (closed) by introducing a VARIABLE_DATA config variable. It's also in PROFILE_ONLY_VARIABLES by default (note that changing it will break a prefix, so generally it should be set when a profile is created, and not changed except with some sort of explicit migration).

Not quite working due to there not currently being a way to access the value in both the sandbox and outside the sandbox (needed in portmodlib.vfs).