Proposal: Permit custom buildCommand to export skaffold ENV vars that could be referenced in later in skaffold.yaml templating

Permit custom buildCommand to export skaffold ENV vars that could be referenced in Helm releases

Would be great if this:
```
build:
artifacts:
- image: myreg.test.com/my/my-app
custom:
buildCommand: ./builder.sh
...
```

Could (via STDOUT or other means) convey some variables that Skaffold could add to its execution environment whereby they could be referenced elsewhere in skaffold.yaml, i.e.

```
deploy:
helm:
releases:
- name: my-app-{{.SOME_VAR_SET_BY_BUILD_CMD}}
chartPath: bitsofinfo-appdeploy/appdeploy
remote: true
version: 1.1.14
...
```

maybe something like if the process has STDOUT that contains something like

```
skaffold.setenvvar.SOME_VAR_SET_BY_BUILD_CMD=myvalue
```
that could be exported into the skaffold environment for reference later on the templating in skaffold.yaml.

Really really could use this!

Azure pipelines has this kind of functionality so this is not unprecedented in the world of cicd.

https://docs.microsoft.com/en-us/azure/devops/pipelines/process/variables?view=azure-devops&tabs=yaml%2Cbatch#set-variables-in-scripts