[develop] Mac Builder Boost Fix

[scottarnette]

Change Description

Added fixes for the following CICD issues:

• Removed leftover SKIP_MOJAVE flag and replaced with flag used in build and test steps: SKIP_MACOS_10_14.
• Fixed issue where Mac pinned builders were not source installing Boost 1.70.
  ** All pinned Mac builders now install Boost 1.70 from source.
  ** All unpinned Mac builders now install Boost via Brew.

See:
https://buildkite.com/EOSIO/eosio/builds/17140 | Pinned
https://buildkite.com/EOSIO/eosio-build-unpinned/builds/515 | Unpinned

Consensus Changes

• Consensus Changes

API Changes

• API Changes

Documentation Additions

• Documentation Additions

[spoonincode]

CMake Warning at /usr/local/lib/cmake/boost_date_time-1.71.0/libboost_date_time-variant-static.cmake:59 (message):
--
  | Target Boost::date_time already has an imported location
  | '/usr/local/lib/libboost_date_time-mt.a', which will be overwritten with
  | '/usr/local/lib/libboost_date_time.a'

I get the impression the build directory may already exist with the contents of an old build when the build script is run?

[scottarnette]

It's interesting because that output doesn't exist for the master or 1.8.x unpinned Mac builds, which use the same builder image since the dependencies are the same:
1.8.x | https://buildkite.com/EOSIO/eosio-build-unpinned/builds/516#288ef056-9721-470c-b159-c1b3e6624b8a/269-290
master | https://buildkite.com/EOSIO/eosio-build-unpinned/builds/512#d6b8fc97-a901-475a-adeb-105b9e8ec4f0/265-286

[scottarnette]

Manual inspection of the Mac builder image showed that no eos directory exists prior to the clone performed by Buildkite.