Due to the `clear_update_cache` parameter not being respected, update caches were being cleared incorrectly which prevented multiple plugins to be updated at the same time in background updates - failing with a `fs_unavailable` error message.
Fixes#38024
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git-svn-id: http://core.svn.wordpress.org/trunk@39151 1a063a9b-81f0-0310-95a4-ce76da25c4cd