Add support for Env in remote shell provisioners

[lbajolet-hashicorp]

Add the support for defining environment variables through the Env map for both: windows-shell, powershell and shell.

Closes #11670

[JenGoldstrich]

This looks good to me Lucas, nice work!

Can you resolve the linting failure before merging?

[nywilken]

This is an interesting find. I believe this is a because we made the decision that new template changes will be optimized for HCL. Env is actually available for both HCL and JSON templates. See an example below. In HCL we don't encourage the use of Go template interpolation. So interpolate is not really a thing. But for JSON some folks may get confused. I think it is okay to let it be as is 🤔

{
  "variables": {
   "sensitive": "I am soooo sensitive"
  },
  "sensitive-variables": ["sensitive"],

	"builders": [
		{
			"type": "null",
			"name": "consul",
			"communicator": "none"
    }
	],
  "provisioners": [
		{
			"type": "shell-local",
			"env": {
			  "TESTNAME": "\"hope\"life"
      },
			"env_var_format": "%s='foo%s' ",
			"inline": ["echo the {{build_name}} and {{build_type}} for $TESTNAME, sensitive var: {{user `sensitive`}}"]
		}
	]
}

[lbajolet-hashicorp]

Oh OK my bad! I assumed that this wouldn't be supported in the legacy JSON templates as they're deprecated and marked as not receiving updates.

If that's not the case, that comment can be removed, and we should definitely interpolate the strings in case that's needed.

Though, and that may be me missing something, I could only see the keys/values interpolated for the Powershell provisioner, and not the shell and windows-shell ones.
Although I'd imagine that'll be fairly rare, I imagine someone may add those, I'd think we should probably fix this in a separate PR.

[nywilken]

Oh no need for apologies. Your assumption is correct, and I think it might be confusing to users as they don't have the code insight we have. That's why its an interesting find.

I agree lets hold for a separate PR. It gives us time to play around and squash and confusion

[lbajolet-hashicorp]

Just so we have a trace for this, shell and windows-shell both interpolate the commands, including the environment definitions, before they upload the resulting script, hence why they don't need to interpolate the values directly in the createFlattenedEnvVars function, as they will be rendered before they are executed in the Provision method.

The powershell provider however does not before it uploads the final script, hence why they're interpolated directly during the flatten phase.
I imagine the rationale is because the approach is different as the variables for Powershell scripts are first uploaded as a script, and then sourced from subsequent scripts instead of being integrated as commands in the prelude.

Note that the rendering could be done in uploadEnvVars from what I see, maybe there's a good reason that it's not done here, we could discuss that later.

[nywilken]

Great work. Appreciate you adding the additional context on the interpolation flow.

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