[Fizz] Always load the external runtime if one is provided (#33091)

Because we now decided whether to outline in the flushing phase, when
we're writing the preamble we don't yet know if we will make that
decision so we don't know if it's safe to omit the external runtime.

However, if you are providing an external runtime it's probably a pretty
safe bet you're streaming something dynamically that's likely to need it
so we can always include it.

The main thing is that this makes it hard to test it because it affects
our tests in ways it wouldn't otherwise so we have to add a bunch of
conditions.