[macOS CI] Bump pinned conda to 26.5.3 to fix osx-arm64 nightly builds (base env now Python 3.14) (#8284)

## Summary

macOS (osx-arm64) nightly wheel builds for the domains (e.g.
torchvision) have failed every night since **2026-07-09** in the
`setup-binary-builds` action. Linux, aarch64-Linux, and Windows are
unaffected.

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 (defaults-channel
builds only cover py3.9–3.13; **26.1.0** is the first release with a
`py314` build):

```
LibMambaUnsatisfiableError: Encountered problems while solving:
  - package conda-25.3.0-py310hca03da5_0 requires python >=3.10,<3.11.0a0, but none of the providers can be installed
...
Pins seem to be involved in the conflict. Currently pinned specs:
 - python=3.14
```

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.

## Fix

Bump the pinned conda to `26.5.3` (verified available on `linux-64`,
`linux-aarch64`, `osx-arm64`, `win-64`).

## Test plan

- Verified `conda=26.5.3` has a `py314` build on `osx-arm64` and exists
on all four platforms that run this action.
- Windows/Linux keep passing (they were already green); this only
unblocks the osx-arm64 base env that moved to Python 3.14.

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