ci: install pinned torch before requirements-ci.txt on macOS (#19342)

### Summary

`setup-macos.sh` runs `install_pip_dependencies` before
`install_pytorch_and_domains`. That order lets torchsr's transitive
torch dep get pulled from PyPI before the pinned source-built or
S3-cached torch lands; `install_pytorch_and_domains` then overwrites the
wrong-source torch.

The overwrite is small in the current state, but the same race forced
--no-cache-dir`. That cascaded reinstalls of every torch transitive dep,
pushed the macOS unittest past its 60-minute timeout, until #19334
reverted it.

Reorder so `install_pip_dependencies` runs after
`install_pytorch_and_domains`. With torch already at the pinned version,
torchsr's dep is satisfied and pip skips re-downloading. Removes the
structural footgun so any future re-land of centralized torch install
does not need `--force-reinstall`.

Also rewrite two adjacent comments:
- Add a comment above `install_pytorch_and_domains` recording the
ordering rationale.
- Drop the now-misleading "We build PyTorch from source here" comment
that drifted above `install_executorch`; replace with one explaining why
`install_executorch --use-pt-pinned-commit` is correct (torch is already
installed).

Authored with Claude Code.

### Test plan
CI

Author: rascani

.ci/scripts/setup-macos.sh

@@ -116,7 +116,6 @@ setup_macos_env_variables
 # buck2 atm
 install_buck
 brew install libomp
-install_pip_dependencies
 
 # TODO(huydhn): Unlike our self-hosted runner, GitHub runner doesn't have access
 # to our infra, so compiler caching needs to be setup differently using GitHub
@@ -125,10 +124,17 @@ if [[ -z "${GITHUB_RUNNER:-}" ]]; then
   install_sccache
 fi
 
+# Install pinned torch before requirements-ci.txt so torchsr's transitive
+# torch dep is satisfied by the existing install and pip does not pull a
+# separate copy from PyPI. sccache is initialized above so source-build
+# cache misses still hit the cache.
 print_cmake_info
 install_pytorch_and_domains
-# We build PyTorch from source here instead of using nightly. This allows CI to test against
-# the pinned commit from PyTorch
+
+install_pip_dependencies
+
+# install_executorch's --use-pt-pinned-commit skips re-installing torch since
+# install_pytorch_and_domains already installed the pinned build above.
 if [[ "$EDITABLE" == "true" ]]; then
   install_executorch --use-pt-pinned-commit --editable
 else

.ci/scripts/utils.sh

@@ -127,6 +127,10 @@ install_pytorch_and_domains() {
   if [[ "${torch_wheel_not_found}" == "1" ]]; then
     echo "No cached wheel found, continue with building PyTorch at ${TORCH_VERSION}"
 
+    # Install PyTorch's own build-time deps so the source build does not
+    # silently inherit them from whatever else happens to be in the env
+    # (e.g. executorch's requirements-ci.txt).
+    pip install -r requirements-build.txt
     git submodule update --init --recursive
     USE_DISTRIBUTED=1 python setup.py bdist_wheel
     pip install "$(echo dist/*.whl)"