ci: Melos root path not needed for running doc setup script (#3340)

It needs to use the full root path to find the script.

scripts/doc-setup.sh

@@ -4,7 +4,7 @@ set -e
 echo Checking python version:
 python3 --version && python3 -c "import sys; sys.exit(0 if sys.version_info >= (3,8) else 2)" || (echo "Error: Python 3.8+ is required" && exit 1)
 echo Installing required python modules:
-python3 -m pip install -r "$MELOS_ROOT_PATH/doc/_sphinx/requirements.txt"
+python3 -m pip install -r "doc/_sphinx/requirements.txt"
 echo Installing dartdoc_json:
 dart pub global activate dartdoc_json
 echo Done.