[macOS CI] Skip conda pin on macOS base env to fix osx-arm64 nightly builds

The macOS binary-build runners' base miniconda env moved to python=3.14
(pinned). `conda install -y conda=25.3.0` is unsatisfiable there because conda
25.3.0 has no Python 3.14 build (py314 builds start at 26.1.0), so every
osx-arm64 nightly wheel build has failed in `setup-binary-builds` since
2026-07-09 with:

    LibMambaUnsatisfiableError: package conda-25.3.0-py310hca03da5_0 requires
    python >=3.10,<3.11.0a0 ... Currently pinned specs: - python=3.14

The macOS base env already ships a newer conda (>=26.1.0) with channels
configured, so it needs no pin. The Linux/Windows build containers, however,
ship an older base env that relies on the implicit `defaults` channel that
conda>=26 removed -- upgrading them to satisfy a floor breaks the subsequent
`conda create` with NoChannelsConfiguredError. So keep the exact `conda=25.3.0`
pin on the containers and skip it on macOS.

This step runs in the base env before the target env is created, which is why
all Python versions failed identically and only osx-arm64 was affected.

Author: atalman

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

@@ -129,7 +129,16 @@ runs:
           set -euxo pipefail
           CONDA_ENV="${RUNNER_TEMP}/conda_environment_${GITHUB_RUN_ID}"
           export CONDA_EXTRA_PARAM=""
-          conda install -y conda=25.3.0
+          # Only pin conda on the Linux/Windows build containers. Their base env
+          # ships an older conda and relies on the implicit `defaults` channel
+          # that conda>=26 removed (upgrading them yields NoChannelsConfiguredError
+          # on the conda create below). The macOS runners' base env is on Python
+          # 3.14 -- which conda=25.3.0 has no build for (py314 starts at 26.1.0) --
+          # and already ships a newer conda with channels configured, so leave it
+          # untouched.
+          if [[ "${RUNNER_OS}" != "macOS" ]]; then
+            conda install -y conda=25.3.0
+          fi
 
           # shellcheck disable=SC2153
           case $PYTHON_VERSION in