ci: Set coverpkg to record dependency code coverage
Created by: keegancsmith
We will now record coverage in all sourcegraph packages. Previously we only recorded the coverage per package test, so missed things like our DB tests covering the migrations bindata.
We used to set this flag, but I think we lost it during our OSS process. Additionally it had a bug that was introduced and then fixed again in go1.13.
Retry of https://github.com/sourcegraph/sourcegraph/pull/4375
Fixes https://github.com/sourcegraph/sourcegraph/issues/4376