ci: fix Windows curl llvm download failure

[lgritz]

For a few days now, the Windows CI variants have been almost always failing due to unexplained inability to curl download the pre-built LLVM that we stashed on a Google drive, which has worked flawlessly for over a year.

J.F. Panisset suggested on Slack:

Could be a wild goose chase, but a direction to look in with your
Windows / curl issue would be SSL certificate verification. Windows
uses some kind of magic process to update its store of recognized CA
root certs, but since a Windows runner is a "fresh" Windows VM every
time, it could be that it is missing an updated cert required to
verify the https URL you are downloading from (assuming that's using
a cert anchored to a root CA not in that base Windows image). I've
seen issues like that on Windows systems with limited Internet
access. If you try curl --insecure to (temporarily) bypass
certificate verification, are you able to download what you need?

And sure enough, adding --insecure clears it right up.

[lgritz]

Merging to fix broken CI

[jfpanisset]

Sorry didn't see this in time for an approval, but yes, that's a good workaround for now.