Re-add `drvPath' and `outPath' attributes to <derivation> XML nodes.
Re-add `drvPath' and `outPath' attributes to <derivation> XML nodes.

This fixes a regression introduced in r20882 ("Add source location
information to the XML output.").

* src/libexpr/expr-to-xml.cc (nix::printTermAsXML): Dereference the
  attribute RHS from "drvPath" and "outPath".

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