guard to exit script if agent is installed

Install-ESETManagementAgent.ps1

@@ -1,5 +1,11 @@
 # NOTE: The $CONFIG variable must be set by the calling script
 
+# Exit this script if agent is already installed
+If ( Test-Path "${Env:ProgramFiles}\ESET\RemoteAdministrator\Agent\ERAAgent.exe" ) {
+    Microsoft.Powershell.Utility\Write-Output "ESET agent is already installed. This script will now exit."
+    Return
+}
+
 # Set path to temp directory
 $TEMPDIR = '{0}eset-protect-deployment' -f (Get-TempPath)