images/kubecross: create a REGISTRY substitution to allow registry selection

[cpanato]

What type of PR is this?

/kind feature

What this PR does / why we need it:

When building the image in another GCP project we need to push the image to the current project and not to the default one that is set in the makefile

this adds an env var to the GCB to set the REGISTRY env variable to be gcr.io/$PROJECT_ID

A previous fix was made but was a mistake because the env var stays in the POD level and not goes to the GCB (xref: kubernetes/test-infra#21307)

/assign @justaugustus

Which issue(s) this PR fixes:

Special notes for your reviewer:

Does this PR introduce a user-facing change?

NONE

[justaugustus]

/hold discussing here: https://kubernetes.slack.com/archives/CJH2GBF7Y/p1615465976127400

[justaugustus]

@cpanato -- You'll need to add this to the substitutions block, as well as variants.yaml, so we respect a user's REGISTRY choice if they decide to push the images to a project that is not the one they built the image in.

[cpanato]

now we have a REGISTRY substitution and the default value is the same as we have in the makefile which points to gcr.io/k8s-staging-build-image

The presubmit job will use the --env-passthrough to set a specific registry.

For other users using this they will need to set this in the variant or when calling the GCB perform the substitution flag

[justaugustus]

@cpanato -- Note the other substitutions in cloudbuild.yaml.

They're purposely set to "bad" values, so a run will fail if substitutions don't happen properly.
Let's do the same for REGISTRY.

[k8s-ci-robot]

@cpanato: The following test failed, say /retest to rerun all failed tests:

Test name	Commit	Details	Rerun command
pull-release-image-kube-cross	d33cbbe	link	/test pull-release-image-kube-cross

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

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[justaugustus]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: cpanato, justaugustus

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[justaugustus]

/hold cancel