Fix platform env var name

Author: loremattei

Example/carthage.sh

@@ -12,7 +12,7 @@ trap 'rm -f "$xcconfig"' INT TERM HUP EXIT
 # For Xcode 12 make sure EXCLUDED_ARCHS is set to arm architectures otherwise
 # the build will fail on lipo due to duplicate architectures.
 echo 'EXCLUDED_ARCHS__EFFECTIVE_PLATFORM_SUFFIX_simulator__NATIVE_ARCH_64_BIT_x86_64__XCODE_1200 = arm64 arm64e armv7 armv7s armv6 armv8' >> $xcconfig
-echo 'EXCLUDED_ARCHS = $(inherited) $(EXCLUDED_ARCHS__EFFECTIVE_PLATFORM_SUFFIX_$(EFFECTIVE_PLATFORM_SUFFIX)__NATIVE_ARCH_64_BIT_$(NATIVE_ARCH_64_BIT)__XCODE_$(XCODE_VERSION_MAJOR))' >> $xcconfig
+echo 'EXCLUDED_ARCHS = $(inherited) $(EXCLUDED_ARCHS__EFFECTIVE_PLATFORM_SUFFIX_$(PLATFORM_NAME)__NATIVE_ARCH_64_BIT_$(NATIVE_ARCH_64_BIT)__XCODE_$(XCODE_VERSION_MAJOR))' >> $xcconfig
 
 export XCODE_XCCONFIG_FILE="$xcconfig"
 carthage "$@"