Fix environment variables to be consistently set when using --system or --permanent (#515)

Also switch PostMessage to SendMessageWithTimeout and only send one if
environment variables actually changed.

Also, enable and fix --permanent unit tests.

.circleci/config.yml

@@ -91,12 +91,7 @@ jobs:
           name: --permanent test
           shell: powershell.exe
           command: |
-            try {
-              scripts/test_permanent.ps1
-            } catch {
-              Write-Host "`n Error Message: [$($_.Exception.Message)"] -ForegroundColor Red
-              Write-Host "`n Line Number: "$_.InvocationInfo.ScriptLineNumber -ForegroundColor Red
-            }
+            scripts/test_permanent.ps1
 
   build-docker-image:
     executor: bionic