osx-x64.yml: Set MACOSX_DEPLOYMENT_TARGET env var to 10.9
This should in theory get picked up by clang automatically and as a result build binaries that work on 10.9 and later. Currently all produced binaries require 10.15.
This should in theory get picked up by clang automatically and as a result build binaries that work on 10.9 and later. Currently all produced binaries require 10.15.