Helm: subcharts are downloaded again at every execution

I'm using Helm. My service needs a DB so I added `postgresql` as a chart dependency.

The subchart is downloaded once (`helm dependency build`) and corresponding `.tgz` file is located in the `charts` subfolder.

However, every time I launch `skaffold dev`, the file is downloaded again.

### Expected behavior

The file is already there, there's nothing to do (regarding subcharts).

### Actual behavior

```
Hang tight while we grab the latest from your chart repositories...
...Successfully got an update from the "bitnami" chart repository
Update Complete. ⎈Happy Helming!⎈
Saving 1 charts
Downloading postgresql from repo https://charts.bitnami.com/bitnami
Deleting outdated charts
```

A subfolder `tmpcharts` appears for less than a second.

### Information

- Skaffold version: 2.0.4 and 2.0.2
- Operating system: Win 10 x64
- Installed via: Chocolatey

### Steps to reproduce the behavior

1. `git clone ... && cd skaffold/examples/helm-deployment`
2. Add the following in `charts\Chart.yaml`:

```yaml
dependencies:
- name: postgresql
version: 12.1.3
repository: https://charts.bitnami.com/bitnami
```

3. `cd charts && helm dependency build && cd ..`
You may need to `helm repo add bitnami https://charts.bitnami.com/bitnami` first.
Observe that `charts\charts\postgresql-12.1.3.tgz` is created.
4. `skaffold dev`
Observe that `postgresql` is downloaded again.