[MERGE #5006 @MSLaguana] Change OSX CI to target 10.12 to fix CI flakiness
[MERGE #5006 @MSLaguana] Change OSX CI to target 10.12 to fix CI flakiness

Merge pull request #5006 from release/1.8-ci

Microsoft/ChakraCore,Microsoft/ChakraCore,Microsoft/ChakraCore,Microsoft/ChakraCore,Microsoft/ChakraCore,Microsoft/ChakraCore