(chocolatey#2044) Fix for changing $env:Temp

Running as `SYSTEM`, the `$env:Temp` variable is normally
*C:\Windows\Temp*. If the **first** package to ever use
`Update-SessionEnvironment.ps1` happens to be running as `SYSTEM`, the
location of `$env:Temp` changes to
*C:\Windows\system32\config\systemprofile\AppData\Local\Temp* for the
duration of the session.  This change to `$env:Temp` does not occur in
subsequent calls to `Update-SessionEnvironment.ps1`.  It is unknown why
this change occurs at all and likewise is unknown why it occurs for only
the first time.

The `Update-SessionEnvironment.ps1` normally gives priority to
user-scope environment variables over machine-scope environment
variables.  Changes in this commit eliminates the gathering of the
`User` scope environment variables when running as the `SYSTEM` account
on the belief that reason to be operating under the `SYSTEM` account is
to do machine-wide activities, and there is no reason to see `SYSTEM`
as a "user".  On a clean system, the only User-scope environment
variables under `SYSTEM` are *path*, *temp*, and *tmp* which are already
defined under the Machine-scope, and in general, there shouldn't be any
new User-scope variables for the SYSTEM account because it's not a
"normal" account.  Basically, there should be little reason to expect
this change to break anything.

src/chocolatey.resources/helpers/functions/Update-SessionEnvironment.ps1

@@ -65,15 +65,18 @@ None
   $architecture = $env:PROCESSOR_ARCHITECTURE
   $psModulePath = $env:PSModulePath
 
-  #ordering is important here, $user comes after so we can override $machine
-  'Process', 'Machine', 'User' |
-    % {
-      $scope = $_
-      Get-EnvironmentVariableNames -Scope $scope |
-        % {
-          Set-Item "Env:$($_)" -Value (Get-EnvironmentVariable -Scope $scope -Name $_)
+  #ordering is important here, $user should override $machine...
+  $ScopeList = 'Process', 'Machine'
+  if ($userName -notin 'SYSTEM', "${env:COMPUTERNAME}`$") {
+    # but only if not running as the SYSTEM/machine in which case user can be ignored.
+    $ScopeList += 'User'
+  }
+  foreach ($Scope in $ScopeList) {
+    Get-EnvironmentVariableNames -Scope $Scope |
+        ForEach-Object {
+          Set-Item "Env:$_" -Value (Get-EnvironmentVariable -Scope $Scope -Name $_)
         }
-    }
+  }
 
   #Path gets special treatment b/c it munges the two together
   $paths = 'Machine', 'User' |