.github: Pin docker buildx version to v0.9.1 (v2)

[joestringer]

GitHub recently rolled out Docker buildx version v0.10.0 on their
builders, which transparently changed the MediaType of docker images to
OCI v1 and added provenance attestations.

Unfortunately, various tools we use in CI like SBOM tooling and docker
manifest inspect do not properly support some aspect of the new image
formats. This resulted in breaking CI, with some messages like this:

level=fatal msg="generating doc: creating SPDX document: generating
SPDX package from image ref quay.io/cilium/docker-plugin-ci:XXX:
generating image package"

This could also lead CI to fail while waiting for image builds to
complete, because the command we use to test whether the image is
available did not support the image types.

This commit attempts to revert buildx back to v0.9.1 to prevent it from
generating the images in a format that other tooling doesn't expect.
Over time we can work on migrating to buildx v0.10, testing various
parts of our CI as we do so.

This is a quick-and-dirty hack to stabilize CI for the short term, then
we can figure out over time how to properly resolve the conflict between
these systems.

Reverts #23206

[ldelossa]

v3's the charm ;)

[joestringer]

Thanks to @chancez for pointing out why the version shouldn't have worked on the previous PR, originally #23206 was trying to update the docker/build-push-action configuration for provenance (which didn't worK). When I switched to configuring version, I thought it was the same action. However, version is only configured on the docker/setup-buildx-action. So hopefully this actually pins the version this time.

[chancez]

Yep looks like the right spot!

[aanm]

Should we revert the SBOM revert PR here?

[joestringer]

@aanm I was hoping to follow up on that separately once we confirmed that this is switching CI back to generating images in the older / supported format

[joestringer]

@aanm #23221

[tommyp1ckles]

@joestringer FYI, you can also work around this by doing docker buildx imagetools inspect --raw quay.io/cilium/cilium-ci:10e55d6053762d098cce06cd97abd51afc6faf0f (credit goes to @pippolo84)

Are we ok to backport this to v1.11, CI is broken over there as well.