[GHA] Use pip-proxy for one more Windows job (#35563)
### Details:
Using `pip-proxy` for Python Unit Tests job on Windows in addition to
Python Layer tests
https://github.com/openvinotoolkit/openvino/pull/34376
This job's modules installation step is the longest and also affected by
`Content-Type: Unknown` error. Let's see if `pip-proxy` helps. There's
one caveat though: currently modules downloaded from
download.pytorch.org registry are not cached.
