Due to swig using a hardcoded swig library path in self-compiled
variants, we need to pass our custom swig library path with every
invocation of swig on macOS.
CMake's `add_custom_command` function does _not_ inherit the environment
variables used during configuration, hence why it needs to be passed
explicitly on invocation.
Usage of CMake's "-E env" command module should allow for cross-platform
compatible invocation.
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