Fix Search::Zoekt::SchedulingService task logic

What does this MR do and why?

This MR fixes https://new-sentry.gitlab.net/organizations/gitlab/issues/628193/?project=3&query=is:unresolved+SchedulingWorker&statsPeriod=14d&stream_index=0

ArgumentError Sidekiq/Search::Zoekt::SchedulingWorker
Unknown task: node_assignment

This happened because sidekiq converts symbol arguments to strings

How to reproduce

[1] pry(main)> Search::Zoekt::SchedulingWorker.new.perform('node_assignment')
ArgumentError: Unknown task: node_assignment
from /Users/dgruzd/projects/gdk-ee/gitlab/ee/app/services/search/zoekt/scheduling_service.rb:22:in `execute'

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