Download rl78.zip from the latest release and extract it to Ghidra/Processors.
Run the following once to set up the build environment:
python3 -mvenv env
. env/bin/activate
pip install -r requirements.txt
Ensure Ghidra's sleigh utility is in your PATH and then execute:
. env/bin/activate
./build.sh
The generated rl78.zip could then be extracted to Ghidra/Processors.