fix: ensure eager effects don't break reactions chain

[dummdidumm]

Execution of eager effects didn't set is_updating_effect, which meant the logic in get would wrongfully prevent dependencies being added to reactions of sources/deriveds.

Fixes #17133
fixes #17130

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🦋 Changeset detected

Latest commit: bc69211

The changes in this PR will be included in the next version bump.

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pnpm add https://pkg.pr.new/svelte@17138

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pnpm add https://pkg.pr.new/svelte@17138

[dylan1951]

Epic! Btw this also fixes #17130