Windows CI Breakages

[Ben Cooksley]

Hi all,

Apologies for the recent crop of CI failures on Windows.

These were caused by Windows Defender's engine eating a significant
proportion of the system memory (to the extent the builders were
swapping to disk even though they have 24GB of RAM available to them)

I have now rebooted the builders and put in place measures to ensure
this issue doesn't recur again (the previous mitigation for this was
broken by the update to Windows 10 version 1908)

Replacement builds have been ordered and should be carried out by the
system once they've caught up with their work queue.

Cheers,
Ben