Setting elevated_user to build user dynamically breaks Powershell provisioner

[InsanesTheName]

Overview of the Issue

There seems to be some kind of templating/interpolation failure when using a build variable as the value for the elevated_user property of the powershell provisioner. The interpolation that is failing appears to be here:

packer-plugin-sdk/guestexec/elevated.go

Line 97 in 7e45483

$f.RegisterTaskDefinition($name, $t, 6, "{{.User}}", $password, $logon_type, $null) | Out-Null

Reproduction Steps

Run a build with the following provisioner:

{
    "type": "powershell",
    "script": "common/test.ps1",
    "environment_vars": [
        "TEST_PACKER_1={{build `User`}}",
        "TEST_PACKER_2={{build `Password`}}"
    ],
    "elevated_user": "{{build `User`}}",
    "elevated_password": "{{build `Password`}}"
}

Interestingly, with the example above, the environment variable TEST_PACKER_1 will be successfully set to the build username (so I don't believe it's an issue with that function), however, the script will fail to launch with the error below. Also, if the username is set explicitly (e.g. "elevated_user": "ExplicitUsername"), everything works fine.

Log Fragments

==> (AWS) Elevated_Test: Provisioning with powershell script: common/test.ps1
==> (AWS) Elevated_Test: (12,8):UserId:
==> (AWS) Elevated_Test: At C:\Windows\Temp\packer-elevated-shell-634090c0-1adf-5fb0-c9b2-82dad6d5bf56.ps1:60 char:1
==> (AWS) Elevated_Test: + $f.RegisterTaskDefinition($name, $t, 6, "{{.User}}", $password, $logo ...
==> (AWS) Elevated_Test: + ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
==> (AWS) Elevated_Test:     + CategoryInfo          : OperationStopped: (:) [], COMException
==> (AWS) Elevated_Test:     + FullyQualifiedErrorId : System.Runtime.InteropServices.COMException

As shown above, the value {{.User}} is being passed into Powershell directly, which is not intended behavior.

Operating system and Environment details

I'm able to reproduce this issue with Packer 1.6.6 and 1.8.3 on both Windows and Linux.
I'm able to reproduce in both amazon-ebs and azure-arm builders.

[InsanesTheName]

bumping this ... can anyone advise?