Enforce dot sourcing. (#33363)

activate.ps1

@@ -1,9 +1,12 @@
 #
-# This file must be used by invoking ". .\activate.ps1" from the command line.
-# You cannot run it directly.
 # To exit from the environment this creates, execute the 'deactivate' function.
 #
 
+if ($MyInvocation.InvocationName -ne '.') {
+    Write-Host -f Red "This script must be dot sourced. Run it by invoking '. .\activate.ps1'."
+    return
+}
+
 function deactivate ([switch]$init) {
 
     # reset old environment variables