CI: Windows fixes, re-enable

Previously, Mesa Windows jobs were restricted to running on an EC2 host with 16GB of RAM. Even with only one job running, this is just too small to link multiple LLVM test jobs all at the same time, given that we can only use static LLVM on Windows. Enabling swapping isn't really an option, as that causes disk thrashing with Windows Defender I've not been able to resolve.

We couldn't run on the GStreamer Hetzner runner as it was previously using an older version of Windows which wasn't ABI-compatible with the containers we were running on it. But that's since been upgraded, so we can use it now, and it should stop OOMing during linking.

This also includes a couple of other small fixes: re-enabling assertions so we can do better testing, and making sure the Meson logs are captured in artifacts.