Admin: Fix plugin search alignment on intermediate viewports.

Add a margin on the plugin installation search form on screens between 1135 and 1250px.

Props jsmansart, hbhalodia, divyeshpatel01, noruzzaman, huzaifaalmesbah, amin7, shailu25, r1k0, joedolson.
Fixes #64809.

git-svn-id: https://develop.svn.wordpress.org/trunk@61941 602fd350-edb4-49c9-b593-d223f7449a82