Security: Don't display the plugin auto-update options for mu-plugins or dropins.

They can't be updated, so we shouldn't be showing the buttons.

Fixes #50459.

Props Chouby, audrasjb, pbiron.


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git-svn-id: https://core.svn.wordpress.org/trunk@47918 1a063a9b-81f0-0310-95a4-ce76da25c4cd