hack: do not set attest flags when exporting to docker

[crazy-max]

Attestation flags should not be set when loading image to docker otherwise:

$ ./hack/images local moby/buildkit
+ BUILDX_NO_DEFAULT_LOAD=true
+ BUILDX_BUILDER=
+ docker buildx build --tag=moby/buildkit:local --output=type=docker,buildinfo-attrs=true --attest=type=sbom --attest=type=provenance,mode=max .
ERROR: docker exporter does not currently support exporting manifest lists

Signed-off-by: CrazyMax crazy-max@users.noreply.github.com

[tonistiigi]

Why do we need to pick this one? Looks like only used in dev?

[crazy-max]

I tried to build a local buildkit image from 0.11 branch and it failed, hence the backport as it's a regression.