Add --cache-file flag to `render` command

I'm using `skaffold render` in my CI/CD GitOps pipeline, and would like to reuse Skaffold cache file between runs, to avoid the dreaded `Checking cache... Not found. Building`. However, I need to write this cache file to a specific location that's not `$HOME/.skaffold/cache`, because that directory is not preserved between my CI steps.

I could move the directory explicitly before/after running `skaffold render`, but it's pretty inconvenient. Other commands (`build`, `dev` etc.) expose `--cache-file` flag to specify its location, and I'd like it to be added to `render` as well, if possible.

Alternatively, if there's any way to set the "Skaffold directory" (from the default `$HOME/.skaffold`), that could work too (and maybe even better!)

### Expected behavior

`skaffold render` has `--cache-file` flag

### Actual behavior
`--cache-file` flag is not available for `skaffold render`

### Information

- Skaffold version: `v1.34.0`
- Operating system: `Ubuntu 20.10`
- Installed via: `Cloud Code`
- Contents of skaffold.yaml: N/A

### Steps to reproduce the behavior

1. Run `skaffold render --cache-file=.skaffold/cache`
2. Observe the result:
```
unknown flag: --cache-file
See 'skaffold render --help' for usage.
```