Add build_wheels_linux_v2.yml: uv-based build (replaces conda)

V2 of build_wheels_linux.yml that provisions Python and the build toolchain
with uv instead of conda/miniforge:
- uv installs the requested Python (incl. free-threaded 3.13t/3.14t and
  pre-releases) and creates a --seed venv activated for all build/test steps.
- pytorch_pkg_helpers (BUILD_ENV_FILE) is run via uv.
- Build tools cmake/ninja/wheel/build installed via uv pip; system libs are
  expected from the manylinux builder image / caller pre-script.
- Same input/secret/output surface as v1 (drop-in), plus an optional
  python-version override; no aarch64 miniforge step.

Needs CI validation across CPU/CUDA/ROCm/XPU + aarch64 before use.

Author: atalman