Allow building using a custom godot-cpp verison #228

Job	Run time
📊 Static Checks (clang-format, black format, file format)	26s
🔧 Build (android, x86_64, android_arch=x86_64, ubuntu-20.04, android-x86_64)	3m 51s
🔧 Build (android, arm64, android_arch=arm64v8, ubuntu-20.04, android-arm64)	3m 26s
🔧 Build (ios, x86_64, ios_arch=x86_64, ios_simulator=true, macos-11, ios-x86_64-simulator)	11m 4s
🔧 Build (ios, arm64, ios_arch=arm64, macos-11, ios-arm64)	10m 32s
🔧 Build (linux, x86_32, i686-godot-linux-gnu_sdk-buildroot, bits=32, ubuntu-20.04, linux-x86_32)	6m 14s
🔧 Build (linux, x86_64, x86_64-godot-linux-gnu_sdk-buildroot, bits=64, ubuntu-20.04, linux-x86_64)	5m 56s
🔧 Build (linux, arm32, arm-godot-linux-gnueabihf_sdk-buildroot, bits=32, ubuntu-20.04, linux-arm32)	6m 13s
🔧 Build (linux, arm64, aarch64-godot-linux-gnu_sdk-buildroot, bits=64, ubuntu-20.04, linux-arm64)	6m 4s
🔧 Build (macos, universal, macos_arch=universal bits=64, macos-11, macos-universal)	19m 9s
🔧 Build (windows, x86_32, bits=32, use_mingw=yes, ubuntu-20.04, amd64_x86, win-x86_32)	7m 21s
🔧 Build (windows, x86_64, bits=64, use_mingw=yes, ubuntu-20.04, amd64, win-x86_64)	6m 44s
📦 Package	1m 41s
	1h 28m 41s

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