Pin MACOSX_DEPLOYMENT_TARGET on the OSS CI macOS

Summary:
The OSS CI macOS legs are failing to compile snapshot_lib/SnapshotReader.cpp
on Xcode 16.4:

  error: 'is_absolute' is unavailable: introduced in macOS 10.15 unknown
    if (src.is_absolute()) { ... }
            ^
  note: 'is_absolute' has been explicitly marked unavailable here
    _LIBCPP_HIDE_FROM_ABI bool is_absolute() const { ... }
                              ^

Cause: the Python.org universal2 frameworks that actions/setup-python@v5
installs for 3.12+ declare a MACOSX_DEPLOYMENT_TARGET of 10.13 in
sysconfig (x86_64 slice). setuptools propagates that into the C++
extension build via -mmacosx-version-min=10.13, which is below the 10.15
floor where libc++ marks std::filesystem::path symbols available, so
the compiler refuses any use of is_absolute(), the path destructor,
etc.

This diff sets MACOSX_DEPLOYMENT_TARGET=11.0 at the build-and-test job
level so the env var is in scope before setup-python runs and before
\`pip install -v .\` builds the extension. 11.0 (rather than the 10.15
strictly required by std::filesystem) matches release.yml's wheel
target (CIBW_ENVIRONMENT_MACOS, line 91), so CI builds against the
same ABI floor as the wheels we publish to PyPI. The env var is
ignored on the Linux matrix legs, so no special-casing is needed.

Reviewed By: pdepetro

Differential Revision: D104851472

fbshipit-source-id: 1a962a4dacf4ad8448ee30b7fbf388a26dd32771

Author: bvu405

.github/workflows/ci.yml

@@ -17,6 +17,14 @@ jobs:
     name: build-and-test (${{ matrix.os }}, py${{ matrix.python-version }})
     runs-on: ${{ matrix.os }}
     timeout-minutes: 30
+    env:
+      # Match release.yml's wheel target (CIBW_ENVIRONMENT_MACOS) so CI builds
+      # the same ABI as published wheels. Also raises the floor above macOS 10.15,
+      # which is required for std::filesystem symbols to be available — the
+      # Python.org 3.12+ universal2 frameworks declare a 10.13 deployment target
+      # in sysconfig, which propagates into extension builds and marks
+      # <filesystem> as unavailable. Ignored on Linux.
+      MACOSX_DEPLOYMENT_TARGET: '11.0'
     strategy:
       fail-fast: false
       matrix: