[macOS CI] Bump pinned conda to 26.5.3 to fix osx-arm64 nightly builds

The macOS binary build runners' base miniconda env now has `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 (builds only cover py3.9-3.13; 26.1.0 is the
first release with a py314 build), 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

Bump the pin to conda=26.5.3 (available on linux-64, linux-aarch64, osx-arm64,
win-64). This 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 +129,10 @@ runs:
           set -euxo pipefail
           CONDA_ENV="${RUNNER_TEMP}/conda_environment_${GITHUB_RUN_ID}"
           export CONDA_EXTRA_PARAM=""
-          conda install -y conda=25.3.0
+          # conda 25.3.0 has no build for Python 3.14, so it cannot be installed
+          # into a base env pinned to python=3.14 (as on the macOS runners now).
+          # 26.1.0 is the first release with a py314 build.
+          conda install -y conda=26.5.3
 
           # shellcheck disable=SC2153
           case $PYTHON_VERSION in