Fix iframe loading when only the iframe server starts on random port

[jonatanklosko]

Closes #2946.

[github-actions]

Uffizzi Ephemeral Environment deployment-61487

☁️ https://app.uffizzi.com/github.com/livebook-dev/livebook/pull/2947

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[jonatanklosko]

When Livebook server port is set to 0, we make the iframe port to be 0, but this is already handled in the config:

livebook/lib/livebook/config.ex

Lines 179 to 184 in c42c5d3

	def iframe_port() do
	case port() do
	0 -> 0
	_ -> Application.fetch_env!(:livebook, :iframe_port)
	end
	end

Here we want to retrieve the actual port, so we should check if the iframe port is 0, regardless of the Livebook port.

The current conditional here fails when Livebook starts on 8080 as expected, but 8081 is taken, in which case we only set the iframe port to 0.

[github-actions]

Uffizzi Preview deployment-61487 was deleted.