preprod: port the codeintel-qa script to this repo
Created by: jhchabran
To ease the maintenance, the main part of this script is ported here, while the preprod part of the script only handles cloning the sourcegraph repo and checking out the right commit.
Test plan
Script itself was already tested on the staging (https://buildkite.com/sourcegraph/deploy-sourcegraph-cloud/builds/204905). As for the plumbing of running this script from the preprod build, that will be tested in the sourcegraph/deploy-sourcegraph-cloud
sister PR.