Build py3.15 wheels against the manylinux python.org interpreter (not conda)

The cp315 segfault is a CPython 3.15 *alpha* ABI skew: torch nightly
cp315 wheels are built with the python.org /opt/python/cp315-cp315
interpreter, but the build env installed python=3.15.0a8 from conda-forge,
whose ABI doesn't match -> 'import torch' segfaults in module init.

For 3.15/3.15t, skip conda and build against /opt/python/cp315-cp315[t]
(the interpreter that produced the torch wheels). pkg-helpers already
prepends that interpreter's bin to PATH, so setting CONDA_RUN='' makes the
whole build/smoke-test use it. Build tooling (cmake/ninja/wheel) is pip
installed; libpng/libjpeg/libwebp/pkg-config come from the manylinux image.
Other versions keep the conda path unchanged.

Author: atalman

.github/actions/setup-binary-builds/action.yml

@@ -129,22 +129,44 @@ runs:
           set -euxo pipefail
           CONDA_ENV="${RUNNER_TEMP}/conda_environment_${GITHUB_RUN_ID}"
           export CONDA_EXTRA_PARAM=""
+
+          # Python 3.15 is still in pre-release and its C-API/ABI is not frozen.
+          # conda-forge's python=3.15.0a* does not match the python.org alpha the
+          # torch nightly cp315 wheels are built against, so importing torch under
+          # the conda interpreter segfaults during module init. Build against the
+          # manylinux python.org interpreter that produced the torch wheels
+          # instead of conda. pkg-helpers already prepends /opt/python/<abi>/bin
+          # to PATH via BUILD_ENV_FILE, so an empty CONDA_RUN makes every
+          # `${CONDA_RUN} python|pip` in the build resolve to that interpreter --
+          # no ABI skew. We only install the build tooling conda used to provide.
+          case $PYTHON_VERSION in
+            3.15) MANYLINUX_PYTHON=/opt/python/cp315-cp315/bin/python ;;
+            3.15t) MANYLINUX_PYTHON=/opt/python/cp315-cp315t/bin/python ;;
+            *) MANYLINUX_PYTHON="" ;;
+          esac
+
+          if [ -n "${MANYLINUX_PYTHON}" ]; then
+            if [ ! -x "${MANYLINUX_PYTHON}" ]; then
+              echo "::error::Expected manylinux interpreter ${MANYLINUX_PYTHON} not found in build image"
+              exit 1
+            fi
+            "${MANYLINUX_PYTHON}" -m pip install --upgrade pip
+            # Match the build-tooling versions pinned for the conda path below.
+            # (libpng/libjpeg/libwebp/pkg-config come from the manylinux image's
+            # system packages rather than conda.)
+            "${MANYLINUX_PYTHON}" -m pip install \
+              "cmake==3.31.2" \
+              "ninja==1.12.1" \
+              "wheel==0.37.1"
+            echo "CONDA_ENV=$(dirname "$(dirname "${MANYLINUX_PYTHON}")")" >> "${GITHUB_ENV}"
+            echo "CONDA_RUN=" >> "${GITHUB_ENV}"
+            exit 0
+          fi
+
           conda install -y conda=25.3.0
 
           # shellcheck disable=SC2153
           case $PYTHON_VERSION in
-            3.15t)
-              # 3.15 is pre-release; conda-forge ships the build under the
-              # python_dev label, but it depends on the _python_rc marker that
-              # only lives on the python_rc label, so both are required. Bump
-              # the pinned alpha/beta as newer pre-releases are published.
-              export PYTHON_VERSION=3.15.0a8
-              export CONDA_EXTRA_PARAM=" python-freethreading -c conda-forge/label/python_dev -c conda-forge/label/python_rc -c conda-forge"
-              ;;
-            3.15)
-              export PYTHON_VERSION=3.15.0a8
-              export CONDA_EXTRA_PARAM=" -c conda-forge/label/python_dev -c conda-forge/label/python_rc -c conda-forge"
-              ;;
             3.14t)
               export PYTHON_VERSION=3.14
               export CONDA_EXTRA_PARAM=" python-freethreading"