Follow-up from "ci: add automated integration branch CI job"

When gitlab#22638 (closed) is merged, let's refactor the create_integration_branch CI job so it reads the package's URL from environment variables rather than passing through file artifacts.

The following discussion from !1007 (merged) should be addressed:

• @pgascouvaillancourt started a discussion: (+1 comment)

  note: We won't need to write the URL to a file anymore once gitlab#22638 (closed) is resolved