Always restore old env variables

When we invoke the script block it could throw an exception so we should
wrap it in a try finally block to ensure that we can restore the old
environment variables, otherwise you can end up back in a shell that has
an incorrect set of variables set.

This is most noticable if an exception is thrown when entering the vs
dev shell since without resetting the old vars you will have invalid
Visual Studio env variables set meaning you need to manually reset your
shell environment.

Author: brianmichel

utils/build.ps1

@@ -799,16 +799,18 @@ function Invoke-IsolatingEnvVars([scriptblock]$Block) {
   foreach ($Var in (Get-ChildItem env:*).GetEnumerator()) {
     $OldVars.Add($Var.Key, $Var.Value)
   }
-
-  & $Block
-
-  Remove-Item env:*
-  foreach ($Var in $OldVars.GetEnumerator()) {
-    New-Item -Path "env:\$($Var.Key)" -Value $Var.Value -ErrorAction Ignore | Out-Null
+  try {
+    & $Block
   }
+  finally {
+    Remove-Item env:*
+    foreach ($Var in $OldVars.GetEnumerator()) {
+      New-Item -Path "env:\$($Var.Key)" -Value $Var.Value -ErrorAction Ignore | Out-Null
+    }
 
-  if ($ToBatch) {
-    Write-Output "endlocal"
+    if ($ToBatch) {
+      Write-Output "endlocal"
+    }
   }
 }