Non Parallel Re-Tagger Sequence

### Expected behavior
Re-tagging existing images in my GCR repo could/should be in parallel

### Actual behavior

It seems to be serialized, I'm not 100% sure if its because each container belongs in its own config / module or always behaves this way

### Information

- Skaffold version: 1.24.1
- Operating system: MacOS
- Installed via: curl
- Contents of skaffold.yaml:

Each Docker container, built on Google Cloud Build, has its own config/module.

Removing the individual container names, but ran with `-v debug` and sha256 tagger

```
INFO[0001] Tags generated in 249.431 micro seconds
.
. *a bunch of containers are found by SHA and retagged to 'latest'
.
INFO[0015] Cache check completed in 13.659 seconds
```