🐛 make release-alias-tag fails on prow image-pushing

[christopherhein]

Step #0: make[3]: Leaving directory '/workspace'
Step #0: make[2]: Leaving directory '/workspace'
Step #0: gcloud container images add-tag gcr.io/k8s-staging-cluster-api-nested/cluster-api-nested-controller:v20210608-ea129c6 gcr.io/k8s-staging-cluster-api-nested/cluster-api-nested-controller:main
Step #0: Created [gcr.io/k8s-staging-cluster-api-nested/cluster-api-nested-controller:main].
Step #0: Updated [gcr.io/k8s-staging-cluster-api-nested/cluster-api-nested-controller:v20210608-ea129c6].
Step #0: gcloud container images add-tag gcr.io/k8s-staging-cluster-api-nested/nested-controlplane-controller:v20210608-ea129c6 gcr.io/k8s-staging-cluster-api-nested/nested-controlplane-controller:main
Step #0: ERROR: Error during upload of: gcr.io/k8s-staging-cluster-api-nested/nested-controlplane-controller:main
Step #0: ERROR: (gcloud.container.images.add-tag) Not found: response: {'status': '404', 'content-length': '202', 'x-xss-protection': '0', 'transfer-encoding': 'chunked', 'server': 'Docker Registry', '-content-encoding': 'gzip', 'docker-distribution-api-version': 'registry/2.0', 'cache-control': 'private', 'date': 'Tue, 08 Jun 2021 18:57:01 GMT', 'x-frame-options': 'SAMEORIGIN', 'content-type': 'application/json'}
Step #0: Failed to fetch "v20210608-ea129c6" from request "/v2/k8s-staging-cluster-api-nested/nested-controlplane-controller/manifests/v20210608-ea129c6".: <no details provided>
Step #0: make[1]: *** [Makefile:353: release-alias-tag] Error 1
Step #0: make[1]: Leaving directory '/workspace'
Step #0: make: *** [Makefile:346: release-staging] Error 2