Remove stray assignment side affect in lambda

This was almost a bug! It wasn't simply because another assignment would
clobber it later.

(cherry picked from commit 32409dd7d7)

src/libstore/derivation-options.cc

@@ -68,7 +68,6 @@ DerivationOptions DerivationOptions::fromParsedDerivation(const ParsedDerivation
                                         throw Error("attribute '%s' must be a list of strings", name);
                                     res.insert(j->get<std::string>());
                                 }
-                                checks.disallowedRequisites = res;
                                 return res;
                             }
                             return {};