Update SDK regex to find macOS 12

[jmagman]

I can't build the engine on Xcode 13.

ERROR at //build/config/mac/mac_sdk.gni:38:7: Script returned non-zero exit code.
      exec_script("//build/mac/find_sdk.py", find_sdk_args, "list lines")
      ^----------
...
Traceback (most recent call last):
  File "/Users/m/Projects/engine/src/build/mac/find_sdk.py", line 101, in <module>
    print(main())
  File "/Users/m/Projects/engine/src/build/mac/find_sdk.py", line 68, in main
    raise Exception('No %s+ SDK found' % min_sdk_version)
Exception: No 10.12+ SDK found

The same issue as #393.