Deprecate unused scripts in Tools/buildbot.
I would just outright delete them, but the readme in PCbuild recommended
their use, so I figure it would be nice to leave them there for a while.
diff --git a/Tools/buildbot/external-amd64.bat b/Tools/buildbot/external-amd64.bat
index 13056c7..bfaef05 100644
--- a/Tools/buildbot/external-amd64.bat
+++ b/Tools/buildbot/external-amd64.bat
@@ -1,2 +1,3 @@
+@echo This script is no longer used and may be removed in the future.
@echo Please use PCbuild\get_externals.bat instead.
@"%~dp0..\..\PCbuild\get_externals.bat" %*