You can use jsonencode to pass complex types to your local-exec commands

variables_complex1.tf

terraform {
  required_version = ">= 1.1.6"
}

locals {
  my_var = "12\" = 1'"
  my_var2 = [
    "Cat",
    "Dog",
    "Cow",
    "Chicken"
  ]
}

resource "null_resource" "complex_example" {
  triggers = {
    always_run = "${timestamp()}"
  }

  provisioner "local-exec" {
    interpreter = ["Powershell", "-Command"]
    environment = {
      MyVar  = local.my_var
      MyVar2 = jsonencode(local.my_var2)
    }
    command = <<EOC
      write-host "my_var value: $Env:MyVar"
      $MyList = $Env:MyVar2 | ConvertFrom-Json

      foreach ($Item in $MyList) {
        Write-Host "The current list item is: $Item"
      }
    EOC
  }
}