Use TLS 1.2 for VBoxGuestAdditions download

The request to virtualbox.org fails without it, and it needs to be
explicitly set in order for Windows Server 2012 R2 to use it.

scripts/vm-guest-tools.bat

@@ -39,7 +39,7 @@ if exist "C:\Users\vagrant\VBoxGuestAdditions.iso" (
 )
 
 if not exist "C:\Windows\Temp\VBoxGuestAdditions.iso" (
-    powershell -Command "(New-Object System.Net.WebClient).DownloadFile('https://download.virtualbox.org/virtualbox/6.0.10/VBoxGuestAdditions_6.0.10.iso', 'C:\Windows\Temp\VBoxGuestAdditions.iso')" <NUL
+    powershell -Command "[Net.ServicePointManager]::SecurityProtocol = [Net.SecurityProtocolType]::Tls12; (New-Object System.Net.WebClient).DownloadFile('https://download.virtualbox.org/virtualbox/6.0.10/VBoxGuestAdditions_6.0.10.iso', 'C:\Windows\Temp\VBoxGuestAdditions.iso')" <NUL
 )
 
 cmd /c ""C:\Program Files\7-Zip\7z.exe" x C:\Windows\Temp\VBoxGuestAdditions.iso -oC:\Windows\Temp\virtualbox"