a8n: Add count:99999 to credentials search query

Created by: mrnugget

This is a small tweak based on the discussion in https://github.com/sourcegraph/sourcegraph/pull/7790#discussion_r367276028 to hopefully avoid users running into the current limitations of our interaction between Automation and Search.

I know that it's hacky because it would break in case the user specifies count: in the scopeQuery, but that is better than us missing results.

I do want to get this into 3.12 as a small cherry-pick.