Prevent starting multiple Capybara proxy servers

What does this MR do and why?

This code is run every time a new Capybara browser session is started. Running this code multiple times causes subsequent feature specs to slow down considerably.

More detailed explanation in gitlab-org/quality/engineering-productivity/master-broken-incidents#6673 (comment 1936109749)

MR acceptance checklist

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How to set up and validate locally

bin/rspec spec/features/issues/user_sees_sidebar_updates_in_realtime_spec.rb spec/features/dashboard/todos/todos_spec.rb

The specs run much slower without this fix.

Related to #461108 (closed)