[CI] Use the latest Python available on Windows
This PR changes our Windows CI to always use the latest Python interpreter available in the GHA tool cache instead of hardcoding Python 3.7.6. This is needed because occasionally GitHub bumps the installed version, deleting the previous one.
This fixes the current GHA outage we're having. I fully expect the outage to propagate to Azure Pipelines in the coming days if we don't merge this, as both GHA and Azure use the same underlying image. Once the PR is merged we can re-enabled the double-gating.
r? @Mark-Simulacrum
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