cloning llvm-project takes 7-9 minutes in CI

[compnerd]

When we run the "Development Snapshot" workflow of https://github.com/thebrowsercompany/swift-build, it takes around 7-9 minutes on average to clone apple/llvm-project on windows-latest. I did a little investigation and it looks like this same clone takes around 1 minute when running on ubuntu-latest. For example: https://github.com/thebrowsercompany/swift-build/actions/runs/8575909350/job/23505928488.

We run this build several times per day and it normally takes around 2 hours to complete. We'd see a roughly 5% speedup if we could make the windows-latest job reach parity with the checkout time on ubuntu-latest. I tried doing a few things but none of them resulted in significantly faster performance. Sharing them here anyway:

Cloning apple/llvm-project on ubuntu-latest and passing the files to subsequent windows-latest jobs using Github's cache.

Same as #1, but using upload-artifact + download-artifact to pass the files between jobs.

Same as #2, but zipping the files before upload & download, and unzipping after.

Disabling Window's Defender's dynamic protection (See this Github issue)

All of these approaches take roughly the same amount of time as the original approach of just git checkouting the project on windows-latest. This leads me to believe that the bottleneck here is performing file I/O on windows, but I haven't confirmed this.