Support CLOUDSDK_GET_CREDENTIALS_OPTS in Helm

## Affected builder image

gcr.io/cloud-builders-community/helm

## Expected Behavior

The CLOUDSDK_GET_CREDENTIALS_OPTS environment variable would be supported to allow passing flags to the 'gcloud container clusters get-credentials' command (specifically '--dns-endpoint' in this case).

## Actual Behavior

This environment variable CLOUDSDK_GET_CREDENTIALS_OPTS is not supported disabling passing flags to the 'gcloud container clusters get-credentials'

## Additional Info

Would like to have the ability to pass flags to the 'gcloud container clusters get-credentials' command in the same way gcr.io/cloud-builders/kubectl supports it (via the environment variable CLOUDSDK_GET_CREDENTIALS_OPTS)