only set cache-control: immutable when appropriate

[Rich-Harris]

follow-up to #3193 — immutable probably doesn't do anything when max-age is 0, but there's still no reason to set it

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🦋 Changeset detected

Latest commit: 076069f

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@sveltejs/adapter-node	Patch

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[Rich-Harris]

abusing my admin powers to merge this one since caching bugs are no joke