ci: upsert license key for backend integration tests
Created by: unknwon
We are going to require a valid license key for repository permissions in https://github.com/sourcegraph/sourcegraph/pull/40826, thus we also need one for backend integration tests as a prerequisite.
Test plan
CI and manually tested, see a verbose run that prints "License key updated": https://buildkite.com/sourcegraph/sourcegraph/builds/170472#0182f759-4988-4594-9f7f-02593bfea2bd/199-446 to prove the SOURCEGRAPH_LICENSE_KEY
is set and not skipped.
cc @miveronese