update CI tarballs for LLVM 18

Also makes the windows CI scripts fetch the tarballs from the network if
not already on disk.

.github/workflows/ci.yaml

@@ -4,6 +4,7 @@ on:
   push:
     branches:
       - master
+      - llvm18
 concurrency:
   # Cancels pending runs when a PR gets updated.
   group: ${{ github.head_ref || github.run_id }}-${{ github.actor }}