Add osx_image to jobs config.

Logs show that the default version of xcode is being used currently. This is possibly because osx_image needs to be configured here instead.

.travis.yml

@@ -104,6 +104,7 @@ jobs:
     # 64-bit OSX build with clang and run tests:
     - if: type != cron AND env(TRAVIS_EVENT_TYPE) != cron
       os: osx
+      osx_image: xcode12
       # gcc on Travis claims to not be CMAKE_COMPILER_IS_GNUCC so we only run clang.
       compiler: clang
       env: DYNAMORIO_CROSS_AARCHXX_LINUX_ONLY=no DEPLOY=no EXTRA_ARGS=64_only