Use a mirror with upload timestamps for PyTorch (#11086)
## Summary
This PR migrates all of our PyTorch tests to use our own mirror, which
includes upload timestamps that we can use to enforce
`--excludes-newer`, making the tests far more stable over time. (Today,
if you checkout old versions of `uv`, many of the PyTorch tests will
fail, since the index contents drift over time.)
Some snapshots changed in this PR (see, e.g.,
`universal_nested_overlapping_local_requirement`). The underlying reason
is that I used the current timestamp when setting upload times in the
PyTorch mirror, but those tests read from both the PyTorch
`--find-links` index _and_ PyPI. I guess we don't omit `--find-links`
entries based on `--excludes-newer`? That might be a bug. But I had to
_increase_ the `--excludes-newer` to include the PyTorch mirror's
`--find-links`, which meant pulling in some newer packages from PyPI
too. This is fine: it's a one-time churn, and they'll be stable going
forward.
