Force a specific target version of macOS.

Otherwise, `delocate` renames the wheels!

Change-Id: I03bf330332a1586deed0519b7983b1a29574ec64
Reviewed-on: https://code-review.googlesource.com/c/re2/+/62911
Reviewed-by: Jeff Bailey <jeffbailey@google.com>
Reviewed-by: Paul Wankadia <junyer@google.com>