fromStructuredAttrs(): Don't crash if exportReferencesGraph is a string

Fixes

  error: [json.exception.type_error.302] type must be array, but is string

and other crashes.

Fixes #13254.

src/libstore/misc.cc

@@ -225,6 +225,8 @@ void Store::queryMissing(const std::vector<DerivedPath> & targets,
             auto parsedDrv = StructuredAttrs::tryParse(drv->env);
             DerivationOptions drvOptions;
             try {
+                // FIXME: this is a lot of work just to get the value
+                // of `allowSubstitutes`.
                 drvOptions = DerivationOptions::fromStructuredAttrs(
                     drv->env,
                     parsedDrv ? &*parsedDrv : nullptr);