different behavior for powershell templatefile script launched through inline or scripts

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#### Overview of the Issue

I was trying to use a script ( with provisioner "powershell" and 'scripts' = ) created from a templatefile and copied to a cdrom with cd_content. The variables were correctly set in the resulting file, but the execution acted like they did not.
Using the exact same script with 'inline' works flawlessly.

### Packer version

1.7.10

#### Reproduction Steps

Create a windows VM with vsphere-iso and add in files the code below.

### Simplified Packer Template

- scripts/bug.ps1

```powershell
$foo = "${foo}"
echo "the value of foo is $foo"
```

- main.pkr.hcl
```
cd_content = {
"bug.ps1" = templatefile("scripts/bug.ps1", {
foo = var.foo
}
)
}
```
```
provisioner "powershell" {
scripts = ["scripts/bug.ps1"]
}
```

```
provisioner "powershell" {
inline = [templatefile("scripts/bug.ps1", {
foo = var.foo
})]
}
```

- variables.pkr.hcl
```
variable "foo" {
type = string
default = "bar"
}
```

### Operating system and Environment details

Gentoo
Bash

### Log Fragments and crash.log files
==> vsphere-iso.windows: Provisioning with powershell script: scripts/bug.ps1
vsphere-iso.windows: the value of foo is
==> vsphere-iso.windows: Provisioning with Powershell...
==> vsphere-iso.windows: Provisioning with powershell script: /tmp/powershell-provisioner1274727291
vsphere-iso.windows: the value of foo is bar

Note that in the first call (scripts) , the value is not printed, while it is in the second one (inline).

The odd thing is that the script is correct on the remote machine ie:
```
$foo = "bar"
echo "the value of foo is $foo"
```
and works fine if manually executed.