gitlab: disable provenance attestations to work around CI bug

QEMU's CI pipeline involves building container images that will be used to run builds and tests. A recent Docker change triggered the following error: $ docker push "$TAG" ... error from registry: blob unknown to registry - sha256:4401f6f779caf8841cafd5f483e642fcac56a23a4e4a59523231e101c890dad9 https://gitlab.com/qemu-project/qemu/-/jobs/15701875927#L2372 This happens because Docker now pushes out-of-order and the GitLab Container Registry rejects due to an unknown reference: https://forum.gitlab.com/t/started-yesterday-docker-push-error-from-registry-blob-unknown-to-registry/134733/5 It is unclear at this point whether GitLab will modify the behavior of Container Registry or whether Docker will ship a fix. The current workaround is to disable the provenance attestation that is involved in this issue. QEMU's CI pipeline container images are used internally for testing and are not widely distributed. Provenance attestation can be disabled as there are no external consumers of these images. Expect to revert this commit in the future when GitLab or Docker have released their own fixes. Cc: Alex Bennée <alex.bennee@linaro.org> Cc: Daniel P. Berrangé <berrange@redhat.com> Cc: Thomas Huth <thuth@redhat.com> Reviewed-by: Daniel P. Berrangé <berrange@redhat.com> Message-ID: <20260804165414.480435-1-stefanha@redhat.com> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>