Re-add core-dev SA secret to preview del job

[vulkoingim]

Description

Re-adds the SA secret used to obtain the core-dev cluster creds. It's required by the delete job to clean-up the certs and LBs.

Currently the job fails:

STDERR: ERROR: (gcloud.auth.activate-service-account) Unable to read file [/mnt/secrets/gcp-sa/service-account.json]: [Errno 2] No such file or directory: '/mnt/secrets/gcp-sa/service-account.json'

https://werft.gitpod-dev.com/job/gitpod-platform-delete-preview-environments-cron-main.2362

Release Notes

NONE

[vulkoingim]

Ran a job from the last commit before the secrets got deleted: https://werft.gitpod-dev.com/job/gitpod-custom-head.16

There seem to be some errors - will run another tomorrow to see if all is good.

Also, there seem to be 2 preview envs on core-dev that fail to get deleted

k --context core-dev get ns
....
staging-as-migrate4d7a06a154   Active   25d
staging-me-test-coredev        Active   25d

Seems like we'll have to clean them up manually :(