Some tickets suggest that the mailsync process doesn't have all environ-
ment variables set which are needed. For example:

https://github.com/LukeSmithxyz/mutt-wizard/issues/678
The error messages indicate that for mailsync, `XDG_DATA_HOME` is empty/
unset and hence it cannot resolve the user's gnupg folder.

This commit defines  those variables via the crontab,  which I think are
the relevant ones. Not sure though if these are all we need or if I for-
got some.