ci(experimental): the Windows leg's LATEST-tier ignore list overflows the command line #2678

Python 3.15 on Windows (experimental) fails before pytest starts, from 61092a529 on: /c/hostedtoolcache/windows/Python/3.15.1/x64/python: Argument list too long, exit 126 (job 125472079249). The windows job's Run unit tests step passes one --ignore=<path> per LATEST-tier module on the command line, and that list has grown past Windows' command-line limit. The commit before it (65a2b4163) still passed. Every later run fails the same way, and the missing junit.xml warning follows from pytest never starting.

Fix: write the ignore list to a file under $RUNNER_TEMP and pass it as @file. pytest's argument parser reads @-prefixed files (fromfile_prefix_chars), and xdist honours the result. test_windows_test_leg_2060.py's argv pin and the Windows-leg paragraph in workflows.rst change with it. Linux legs are not affected.