(GH-188) Filter current environment variables

When Protected Event Logging and PowerShell Script Block logging are
enabled together the SystemRoot environment variable is a requirement.

If it is removed as part of the environment purge it causes the
PowerShell process to crash, therefore breaking the pipe between Ruby
and the remote PowerShell session.

This commit fixes the issue by creating a filtered list of environment
variables which does not include the SystemRoot environment variable.

This list can then be purged safely.

Author: chelnak

lib/templates/init.ps1

@@ -35,8 +35,17 @@ $global:ourFunctions = @'
 function Reset-ProcessEnvironmentVariables {
   param($CachedEnvironmentVariables)
 
+  # When Protected Event Logging and PowerShell Script Block logging are enabled together
+  # the SystemRoot environment variable is a requirement. If it is removed as part of this purge
+  # it causes the PowerShell process to crash, therefore breaking the pipe between Ruby and the
+  # remote PowerShell session.
+  # The least descructive way to avoid this is to filter out SystemRoot when pulling our current list
+  # of environment variables. Then we can continue safely with the removal.
+  $CurrentEnvironmentVariables = Get-ChildItem -Path Env:\* |
+    Where-Object {$_.Name -ne "SystemRoot"}
+
   # Delete existing environment variables
-  Remove-Item -Path Env:\* -ErrorAction SilentlyContinue -WarningAction SilentlyContinue -Recurse
+  Remove-Item -Path $CurrentEnvironmentVariables -ErrorAction SilentlyContinue -WarningAction SilentlyContinue -Recurse
 
   # Re-add the cached environment variables
   $CachedEnvironmentVariables |