Install "six" in CT Win GCE instances

Bug: skia:11888
Change-Id: I80d1c89da69d382c5b799c563bf16668e634a8a5
Reviewed-on: https://skia-review.googlesource.com/c/buildbot/+/398940
Reviewed-by: Eric Boren <borenet@google.com>
Commit-Queue: Ravi Mistry <rmistry@google.com>
diff --git a/scripts/chromebot-schtask.ps1 b/scripts/chromebot-schtask.ps1
index f1f3e9b..6a971f4 100644
--- a/scripts/chromebot-schtask.ps1
+++ b/scripts/chromebot-schtask.ps1
@@ -181,6 +181,8 @@
   cmd /c "C:\Python27\Scripts\pip.exe install -U psutil"
   banner "Installing requests using pip. Required for CT win bots (skbug/10960)."
   cmd /c "C:\Python27\Scripts\pip.exe install -U requests"
+  banner "Installing six using pip. Required for CT win bots (skbug/11888)."
+  cmd /c "C:\Python27\Scripts\pip.exe install -U six"
 }
 if ($hostname.StartsWith("ct-windows-builder")) {
   banner "Check out Chromium repository"