fix(backend): expand vars for rc and data location

[npoirot]

When passing an environment variable such as $XDG_CONFIG_HOME or $XDG_DATA_HOME to a TaskWarrior instance of the library, the variables are not expanded leading to ignoring rc_location or creating a path like '~/$XDG_DATA_HOME'.

Encountered this behavior using taskwiki vim plugin which does not expand any variables either.