ENV: don't set MACOSX_DEPLOYMENT_TARGET

[ilovezfs]

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CMake tries to use Xcode if MACOSX_DEPLOYMENT_TARGET is set but that can
lead to build failures when SDKROOT isn't set. The CMake behavior at
minimum manifests as -isysroot spontaneously being set to the Xcode SDK,
which brew sometimes can't successfully fully unwind with its ENV hacks.