sproxy2: fix `datafile` option processing
sproxy2: fix `datafile` option processing

`datafile` is usually a derivation (a set at the very basic
nix level), so it wasn't processed as a string, but rather
as a fallback case "toString v".

Other option is to use `lib.isDerivation`.

ip1981/nixsap,ip1981/nixsap,ip1981/nixsap,zalora/nixsap,zalora/nixsap