BuildMacOSUniversalBinary: Explicitly specify CMAKE_OSX_DEPLOYMENT_TARGET in CMake flags

If the build folder is created from an older commit and this flag is not set, the last value set for this flag will be used instead.
This is currently happening on our macOS build machine, causing macOS 10.15 to still be used as the deployment target.

BuildMacOSUniversalBinary.py

@@ -288,6 +288,7 @@ def build(config):
                 "-DCMAKE_PREFIX_PATH="+prefix_path,
                 "-DCMAKE_SYSTEM_PROCESSOR="+arch,
                 "-DCMAKE_IGNORE_PATH="+ignore_path,
+                "-DCMAKE_OSX_DEPLOYMENT_TARGET=11.0.0",
                 "-DMACOS_CODE_SIGNING_IDENTITY="
                 + config["codesign_identity"],
                 '-DMACOS_CODE_SIGNING="ON"',