Fix Web Project Export Download rate limiting scope

What does this MR do?

This MR fixes a bug in Web (Controller) Project Export rate limiting, where incorrect scope was applied for export download.

From #323637 (closed)

project_scope = params[:action] == :download_export ? @project : nil return nil because params[:action] is a string and not a symbol.

   522:     prefixed_action = "project_#{params[:action]}".to_sym
   523:
   524:     project_scope = params[:action] == :download_export ? @project : nil
   525: 
   526:     require 'byebug'; byebug
   527: 
=> 528:     if rate_limiter.throttled?(prefixed_action, scope: [current_user, project_scope].compact)
   529:       rate_limiter.log_request(request, "#{prefixed_action}_request_limit".to_sym, current_user)
   530: 
   531:       render plain: _('This endpoint has been requested too many times. Try again later.'), status: :too_many_requests
   532:     end

=> params[:action]

"download_export"

Making it a string fixes this issue, as it does not have to be a symbol. Only the action itself is required to be one according to Gitlab::ApplicationRateLimiter.throttled? documentation.

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