Plugins: Update the height of the Install Now button for new plugins.

Due to Thickbox shenanigans, the buttons height needed to be shrunk a little to center properly in the iframe.

Fixes #49828.

Props ibachal, Otto42, afercia.

Built from https://develop.svn.wordpress.org/trunk@48503


git-svn-id: https://core.svn.wordpress.org/trunk@48265 1a063a9b-81f0-0310-95a4-ce76da25c4cd