Follow XDG Base Directory spec for gvlsbuild
If ~/.gvls doesn't exist and GVLS_CONFIG wasn't set as environmental variable, use $XDG_DATA_HOME/gvls instead of hardcoding ~/.gvls. If ~/.gvls already exists though, keep using it.
If ~/.gvls doesn't exist and GVLS_CONFIG wasn't set as environmental variable, use $XDG_DATA_HOME/gvls instead of hardcoding ~/.gvls. If ~/.gvls already exists though, keep using it.