Use the variant's image in the AutoEncryption verify job

The state-is-encrypted job in the AutoEncryption integration test always pulled the alpine image. The debian variant of the test only waits for the debian image build, so the job failed whenever it started before the alpine image was pushed. That happened in the pipeline of !599 (merged) (job: manifest … -opentofu1.13.9-alpine not found).

The job now uses the image of its own variant through $GITLAB_OPENTOFU_BASE_IMAGE_OS, which the same child pipeline already passes to the apply and delete-state components. No other integration test hardcodes the image OS.