hack/build-and-push.sh: fix digest existence check #904

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PipelineRun: <a href="https://console.redhat.com/preview/application-pipeline/ns/tekton-ci/pipelinerun/build-definitions-pull-request-h6wqt" rel="nofollow">build-definitions-pull-request-h6wqt

Success

Red Hat Konflux/build-definitions-pull-request has successfully validated your commit.

Status	Duration	Name
✅ Succeeded	27 seconds	fetch-repository
✅ Succeeded	29 seconds	yaml-lint-check
✅ Succeeded	31 seconds	sast-snyk-check
✅ Succeeded	1 minute	ec-task-checks
✅ Succeeded	27 seconds	check-task-migration-md
✅ Succeeded	2 minutes	build-container
✅ Succeeded	1 minute	check-partner-tasks
✅ Succeeded	1 minute	build-bundles
✅ Succeeded	1 hour	e2e-tests
✅ Succeeded	52 seconds	check-task-pipeline-repo-existence
✅ Succeeded	6 seconds	e2e-cleanup