Switch off KUBE_BUILD_CONFORMANCE to diagnose a ci-cross issue

[dims]

What this PR does / why we need it:

Looking at job results, there seems to be some flakiness, let's see if
the PR that added the conformance image is the one to blame.

https://prow.k8s.io/?type=periodic&job=ci-kubernetes-cross-build
https://k8s-testgrid.appspot.com/sig-release-misc#cross-build&graph-metrics=test-duration-minutes&width=20

Change-Id: Ic26431a0e262100dec0444279d94277482972b07

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

Special notes for your reviewer:

Release note:

NONE

[dims]

/assign @spiffxp
/sig release
/kind bug

[dims]

/assign @cblecker

[dougm]

/lgtm

thanks @dims, I'll start another alpha.1 build once this has merged.

[spiffxp]

/lgtm

[cblecker]

/approve

Looks like this is a new option that was added in #69368, so I'm comfortable turning it off.

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: cblecker, dims

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[dims]

/test pull-kubernetes-integration

[ixdy]

It looks like the root cause is that both the hyperkube and conformance image Makefiles are trying to run docker run --rm --privileged multiarch/qemu-user-static:register --reset at the same time. If they're called sequentially, there should be no issue, but if they're called at the exact same time, they'll race with each other and (at least) one of them will fail.

[dims]

ah wow! that's great bit of analysis @ixdy

[dims]

@ixdy we could use flock? (example https://www.msully.net/blog/2015/02/25/parallelizing-compiles-without-parallelizing-linking-using-make/ )

[ixdy]

flock should work, though I'm not sure if that'd cause problems for macOS builds, since I think flock is a linux-specific utility.