ci: fix macOS PyTorch wheel cache key for branch-ref pins

`install_pytorch_and_domains` constructs the cached-wheel URL using
`${TORCH_VERSION:0:7}`, which gives "release" when the pin is a
branch ref like `release/2.11`. The upload code uses the basename of
`dist/*.whl`, which is whatever PyTorch's setup.py wrote — always the
resolved commit hash (e.g. `+git70d99e9`). The two never match, so
every macOS run misses the cache and does a ~30-minute source build
even though the wheel for the current pin's HEAD is already in S3.

Resolve the hash via `git rev-parse --short=7 HEAD` after `git
checkout`, so download and upload agree. Commit-hash pins are
unchanged (the first 7 chars already equaled the resolved hash).

Authored with Claude Code.

Author: rascani

.ci/scripts/utils.sh

@@ -105,7 +105,11 @@ install_pytorch_and_domains() {
   fi
   local python_version=$(python -c 'import platform; v=platform.python_version_tuple(); print(f"{v[0]}{v[1]}")')
   local torch_release=$(cat version.txt)
-  local torch_short_hash=${TORCH_VERSION:0:7}
+  # Download key must match the upload key below (basename of dist/*.whl,
+  # which always carries setup.py's resolved +gitHASH). Branch-ref pins
+  # like `release/2.11` would otherwise produce `+gitrelease` here and
+  # never hit the cache.
+  local torch_short_hash=$(git rev-parse --short=7 HEAD)
   local torch_wheel_path="cached_artifacts/pytorch/executorch/pytorch_wheels/${system_name}/${python_version}"
   local torch_wheel_name="torch-${torch_release}%2Bgit${torch_short_hash}-cp${python_version}-cp${python_version}-${platform:-}.whl"