fix not saving to "XDG_DATA_HOME", when "XDG_DATA_DIRS" is available.
fix "Content Store" persist in flatpak.
XDG specification specifies how to find config and data directories on
linux systems. Specifically, it says this:
- Data should be written to $XDG_DATA_HOME
- Config should be written to $XDG_CONFIG_HOME
- Data should be read from $XDG_DATA_HOME:$XDG_DATA_DIRS
- Config should be read from $XDG_CONFIG_HOME:$XDG_CONFIG_DIRS
3b8b95a22f