Unable to pull helm dependencies from a private helm repo

### Expected behavior
I have a local helm chart which includes some remote deps from a private helm repo. I run skaffold through Google Cloud Deploy.
I expect skaffold was able to get authenticated to the helm private repo to pull the dependencies.

### Actual behavior
I get this error message
`building helm dependencies: exit status 1`

### Information

- Skaffold version: 2.11.0
- Operating system: no OS. Google Cloud Deploy
- Installed via: Google Cloud Deploy
- Contents of skaffold.yaml:

```yaml
apiVersion: skaffold/v4beta7
kind: Config
profiles:
- name: preprod
deploy:
helm:
releases:
- name: foo
createNamespace: true
namespace: default
chartPath: ./helm
repo: https://example.com/repository/helm
skipBuildDependencies: false
valuesFiles:
- envs/preprod/values.yaml
setValueTemplates:
image.tag: "0.0.1"
- name: prod
deploy:
helm:
releases:
- name: foo
createNamespace: false
namespace: default
chartPath: ./helm
repo: https://example.com/repository/helm
valuesFiles:
- envs/prod/values.yaml
setValueTemplates:
image.tag: "0.0.1"
```