Allow templating for buildpack volumes

### Expected behavior
Templating for buildpack volumes work

### Actual behavior
Templating for buildpack volumes doesn't work

### Information

- Skaffold version: v2.0.3
- Operating system: macOS
- Installed via: Homebrew
- Contents of skaffold.yaml:

This is a java project we are trying to build with maven

```yaml
apiVersion: skaffold/v4beta1
kind: Config
metadata:
name: foo
profiles:
- name: local
build:
local:
concurrency: 1
tryImportMissing: false
useDockerCLI: false
artifacts:
- image: foo.dkr.ecr.eu-west-1.amazonaws.com/myapp/demo
buildpacks:
clearCache: false
trustBuilder: false
builder: paketobuildpacks/builder:tiny
env:
- ARTIFACTORY_USERNAME={{.ARTIFACTORY_USERNAME}}
- ARTIFACTORY_PASSWORD={{.ARTIFACTORY_PASSWORD}}
volumes:
- host: {{.PWD}}/platform/bindings/maven
target: /platform/bindings/maven
manifests:
rawYaml:
- ./k8s/*.yaml
deploy:
kubectl: {}
kubeContext: kind-kind
```

### Steps to reproduce the behavior

1. Use the yaml above
2. Start a kind cluster
3. `skaffold run -p local`
4. See that the buildpack is not able to resolve the templated value