Make `deactivate` clean x86 root (#36759)

- backport of #36717 to release/6.0

Co-authored-by: Doug Bunting <6431421+dougbu@users.noreply.github.com>

activate.ps1

@@ -25,6 +25,7 @@ function deactivate ([switch]$init) {
     }
 
     Remove-Item env:DOTNET_ROOT -ea ignore
+    Remove-Item 'env:DOTNET_ROOT(x86)' -ea Ignore
     Remove-Item env:DOTNET_MULTILEVEL_LOOKUP -ea ignore
     if (-not $init) {
         # Remove the deactivate function