getdeps: emit DEVELOPER_DIR for macOS CI builds; regenerate rebalancer workflow

Summary:
Effectively bumps XCode to 16.2 - which has better support for modern C++.

macOS GitHub Actions runners with multiple Xcode versions installed need DEVELOPER_DIR set to select the correct toolchain. Without it, CMake may find the wrong compiler or SDK and the build can fail. Add DEVELOPER_DIR: /Applications/Xcode_16.2.app/Contents/Developer to the env block emitted for darwin builds in the getdeps.py generate-github-actions command, and regenerate the rebalancer oss_root/.github/workflows/getdeps_mac.yml to match.

Reviewed By: xiatingouyang

Differential Revision: D102531661

fbshipit-source-id: 62ac61a44989414e59df5d7a50a166548c354287

Author: r-barnes

build/fbcode_builder/getdeps.py

@@ -1179,9 +1179,17 @@ def write_job_for_platform(self, platform, args):  # noqa: C901
             out.write("  build:\n")
             out.write("    runs-on: %s\n" % runs_on)
             out.write(f"    timeout-minutes: {timeout_minutes}\n")
+            env_vars = []
+            if build_opts.is_darwin():
+                env_vars.append(
+                    "      DEVELOPER_DIR: /Applications/Xcode_16.2.app/Contents/Developer\n"
+                )
             if use_sccache:
+                env_vars.append('      SCCACHE_GHA_ENABLED: "on"\n')
+            if env_vars:
                 out.write("    env:\n")
-                out.write('      SCCACHE_GHA_ENABLED: "on"\n')
+                for line in env_vars:
+                    out.write(line)
             out.write("    steps:\n")
 
             if build_opts.is_linux():