CI: fix flapping Sidekiq test

[fallwith]

We have a unit test to confirm that we properly record Sidekiq "internal" errors (errors that take place within Sidekiq's own handling of things and not job specific). That unit test performs the following:

Sidekiq::CLI.instance.handle_exception(exception)

Occasionally - and apparently only via our GitHub Actions runners and not locally - the instance yielded is nil, which doesn't define #handle_exception.

We should see if there's a way to guard against this error with a retry or whatnot.

  1) Error:
SidekiqInstrumentationTest#test_captures_sidekiq_internal_errors:
NoMethodError: undefined method `handle_exception' for nil:NilClass

      config.handle_exception(ex, ctx)
            ^^^^^^^^^^^^^^^^^
    /the/ruby/path/Ruby/3.1.4/x64/lib/ruby/gems/3.1.0/gems/sidekiq-7.1.4/lib/sidekiq/component.rb:48:in `handle_exception'
    /the/runner/path/newrelic-ruby-agent/newrelic-ruby-agent/test/multiverse/suites/sidekiq/sidekiq_instrumentation_test.rb:50:in `block in test_captures_sidekiq_internal_errors'

[workato-integration]

https://issues.newrelic.com/browse/NR-162505