Change workspace integration tests nightly from core-dev to preview environment

[jenting]

Bug description

The platform team recently (May 2) change the default environment as preview environment, deprecated the core-dev.
https://gitpod.slack.com/archives/C032A46PWR0/p1651473669803169

Therefore, the nightly integration test can't run successfully.

Steps to reproduce

Run the werft job

werft run github -j .werft/workspace-integration-tests-startup-cron.yaml

Workspace affected

No response

Expected behavior

No response

Example repository

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Anything else?

No response