Fix fatal error in SwapcardQueueWorkerRequest due to missing global namespace for Drush function

Illia Stankevychrequested to merge
issue/swapcard-3522804:3522804-undefined-function-drushbackendbatchprocess into 1.1.x

The call to drush_backend_batch_process() caused a fatal error because it was not prefixed with a backslash. This caused PHP to look for the function within the class namespace. The call has been updated to \drush_backend_batch_process() to ensure the global function is used.

Closes #3522804

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