fix: don't mark deriveds as clean if updating during teardown

[Rich-Harris]

Fixes the issue reported here: https://www.reddit.com/r/sveltejs/comments/1ku7skr/can_someone_explain_this_weird_behavior_i_really. Previously, deriveds read during effect teardown were being marked clean, which caused them to hold onto incorrect values if they depended on state that had just changed

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