Ensure Docker experimental features are enabled before creating manifest

[ixdy]

What type of PR is this?
/kind bug

What this PR does / why we need it: When building and pushing new e2e images, make all-push can fail to create/push the manifest list if experimental features are not enabled in the Docker client.

We've fixed this in some of the other Makefiles by setting the DOCKER_CLI_EXPERIMENTAL environment variable, so let's do this in image-util.sh as well.

Does this PR introduce a user-facing change?:

NONE

/sig testing
/priority important-soon
/assign @mkumatag
cc @jiayingz

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This pull-request has been approved by: ixdy

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[ixdy]

/retest

[mkumatag]

/lgtm