invalid skaffold config: proxy: unknown scheme: http

I followed the getting started guide (using skaffold v2.10.0), i.e.:

```shell
git clone https://github.com/GoogleContainerTools/skaffold
cd skaffold/examples/buildpacks-node-tutorial
skaffold init
minikube start --profile custom
skaffold config set --global local-cluster true
eval $(minikube -p custom docker-env)
```

Once I call `skaffold dev` I get:

> invalid skaffold config: proxy: unknown scheme: http

I am behind a corporate proxy.

If I do

```shell
unset ALL_PROXY
unset all_proxy
```

the issue is gone.

Would be nice if skaffold (or its transitive dependencies) would be less pedantic here.