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


git-svn-id: https://develop.svn.wordpress.org/trunk@25350 602fd350-edb4-49c9-b593-d223f7449a82