ci(e2e): fix GitLab e2e push attestations

[crazy-max]

relates to https://github.com/docker/build-push-action/actions/runs/30906625924/job/91983111633#step:12:406

#41 [auth] test1716/test:pull,push token for registry.gitlab.com
#41 DONE 0.0s

#40 exporting to image
#40 pushing layers 6.2s done
#40 pushing manifest for registry.gitlab.com/test1716/test:gh-runid-30906625924@sha256:24315ad91bbd1adaccda5231791fbb02d9f3ee36800907359411b7c7cdba1719
#40 pushing manifest for registry.gitlab.com/test1716/test:gh-runid-30906625924@sha256:24315ad91bbd1adaccda5231791fbb02d9f3ee36800907359411b7c7cdba1719 0.6s done
#40 ERROR: failed to push registry.gitlab.com/test1716/test:gh-runid-30906625924: unknown: blob unknown to registry - sha256:63adb36080a3ab14b9f318345734e94f75d748cb415475e941473177d541f726
------
 > exporting to image:
------
ERROR: failed to build: failed to solve: failed to push registry.gitlab.com/test1716/test:gh-runid-30906625924: unknown: blob unknown to registry - sha256:63adb36080a3ab14b9f318345734e94f75d748cb415475e941473177d541f726

This is a follow-up to docker/buildx#3991. It applies the new Buildx compatibility escape hatch to the GitLab e2e registry runs.

The e2e reusable workflow now sets BUILDX_NO_DEFAULT_OCI_ARTIFACT=true only when the provider is GitLab. The workaround is documented with a FIXME so it can be removed once GitLab Registry accepts OCI artifact attestations whose subject references a manifest pushed in the same build.

GitLab Registry can reject pushes that use the newer OCI artifact attestation storage format and fail with blob unknown to registry. Setting this environment variable keeps provenance attestations enabled while making Buildx use the legacy attestation storage format for GitLab.

[thaJeztah]

LGTM