Fix the action that `set_site_transient()` fires so as not to include the private option prefix. This brings set_site_transient() back in line with it's documented behaviour, and the behaviour of all other transient functions. Fixes #25213

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git-svn-id: http://core.svn.wordpress.org/trunk@25312 1a063a9b-81f0-0310-95a4-ce76da25c4cd