ci: update Kokoro root directory (#882)
ci: update Kokoro root directory (#882)

To run nightly samples resource cleanup jobs.

Still observing:
```
ERROR: (gcloud.auth.activate-service-account) Unable to read file [/tmpfs/src/gfile/service-acct.json]: [Errno 2] No such file or directory: '/tmpfs/src/gfile/service-acct.json'
```

https://g3c.corp.google.com/results/invocations/cc95e6ae-5986-430d-b69e-ef87696ba02c/targets/cloud-devrel%2Fclient-libraries%2Fjava%2Fjava-bigquery%2Fnightly%2Fsamples/log
googleapis/java-bigquery,googleapis/java-bigquery,googleapis/java-bigquery