Set MACOSX_DEPLOYMENT_TARGET to be the earliest supported version

[bjia56]

The MACOSX_DEPLOYMENT_TARGET env var controls which minimum MacOS version the compilation will target. For arm64, the minimum possible version is always MacOS 11.0 (even if the env var is set to be lower), but for x86_64 the version could be even lower (e.g. 10.15).

Without setting this env var, libcurl may not work on users running an older MacOS than the gh actions worker - e.g. in this issue: jeffreydwalter/arlo#205