pin the macOS release job back to macOS-10.15 to continue targeting Big Sur
pin the macOS release job back to macOS-10.15 to continue targeting Big Sur

Summary:
When macOS-latest got bumped to 11, the default Xcode and its
available SDKs changed. For now, pin the macOS release job to the
older image.

Reviewed By: genevievehelsel

Differential Revision: D34692830

fbshipit-source-id: 9c02b8e8acef37820c08b521ed433efbb18aee6c

facebook/watchman,facebook/watchman,nodakai/watchman,nodakai/watchman,facebook/watchman,facebook/watchman,facebook/watchman,facebook/watchman,facebook/watchman,nodakai/watchman,nodakai/watchman,nodakai/watchman,facebook/watchman,nodakai/watchman,facebook/watchman,nodakai/watchman,nodakai/watchman,nodakai/watchman