`skaffold render --filename -` breaks due to helm post renderer

Skaffold render breaks when trying to render manifests from helm charts, when the Skaffold config is passed via `stdin`.
The issue seems to be that Helm post renderer expects the rendered manifests on `stdin`, while Skaffold expects the configuration on `stdin` as well.

Our use-case for using `--filename -` is pre-processing of the configuration, to e.g. replace env vars on un-templated fields and such.
While it would be possible to store the intermediate config in a temporary file, it is inconvenient, especially when running locally.

### Expected behavior

Piping a valid configuration to `skaffold render --filename -` renders valid manifests according to the configuration (helm releases).

### Actual behavior

```
std out err: %!(EXTRA *errors.errorString=Error: error while running post render on files: error while running command /opt/homebrew/bin/skaffold. error output:
parsing skaffold config: error parsing skaffold configuration file: unknown skaffold config API version "v1". Set the config 'apiVersion' to a known value. Check https://skaffold.dev/docs/references/yaml/ for the list of valid API versions. Otherwise, check that your skaffold version is up-to-date.
: exit status 1
```

### Information

- Skaffold version: `2.5.0`
- Operating system: `macos 12.6.5`
- Installed via: Homebrew
- Contents of skaffold.yaml:

```yaml
apiVersion: skaffold/v4beta5
kind: 'Config'
build:
artifacts:
- image: 'temp/one'
manifests:
helm:
releases:
- name: 'one'
chartPath: 'helm/charts/test'
setValueTemplates:
image.tag: 'test@{{.IMAGE_DIGEST}}'
image.repository: 'temp/one'
```

### Steps to reproduce the behavior

1. a clonable repository with the sample skaffold project and a helm chart to render (`helm create helm/charts/test`)
2. run `cat skaffold.yaml | skaffold render --filename - --digest-source tag`
3. observe error