Error when there's charts directory in my chart

get errors when there's charts directory in my chart.

```
 mkdir: /var/folders/sd/l671vt950cs2qmj9xkdhprzw0000gp/T/tmp.hMGRKC8g/nginx/charts: File exists
```

[ "mkdir $CHART/charts",](https://github.com/tnarg/rules_helm/blob/master/helm/charts.bzl#L55)