fix missing variable and add explanations #7516

Job	Run time
macOS-12 - setuptools (CC=gcc-9 CXX=g++-9)	9m 54s
macOS-12 - cmake (CC=gcc-10 CXX=g++-10)	12m 39s
macOS-12 - cmake (CC=gcc-10 CXX=g++-10)	14m 55s
macOS-12 - cmake (-DNRN_ENABLE_CORENEURON=ONCC=gcc-9 CXX=g++-9)	13m 23s
macOS-12 - cmake (-DNRN_ENABLE_MPI=OFF -DNRN_ENABLE_INTERVIEWS=OFF -DNRN_ENABLE_CORENEURON=ONCC=gcc-9 CXX=g++-9)	13m 2s
macOS-12 - cmake (-DNRN_ENABLE_PYTHON=OFF -DNRN_ENABLE_RX3D=OFF -DNRN_ENABLE_CORENEURON=ONCC=gcc-10 CXX=g++-10)	8m 39s
ubuntu-20.04 - setuptools (CC=gcc-9 CXX=g++-9)	6m 16s
ubuntu-20.04 - cmake (CC=gcc-10 CXX=g++-10)	8m 12s
ubuntu-20.04 - cmake (CC=gcc-10 CXX=g++-10)	9m 48s
ubuntu-20.04 - cmake (-DNRN_ENABLE_CORENEURON=ONCC=gcc-9 CXX=g++-9)	8m 56s
ubuntu-20.04 - cmake (-DNRN_ENABLE_MPI=OFF -DNRN_ENABLE_INTERVIEWS=OFF -DNRN_ENABLE_CORENEURON=ONCC=gcc-9 CXX=g++-9)	6m 45s
ubuntu-20.04 - cmake (-DNRN_ENABLE_PYTHON=OFF -DNRN_ENABLE_RX3D=OFF -DNRN_ENABLE_CORENEURON=ONCC=gcc-10 CXX=g++-10)	4m 18s
ubuntu-22.04 - cmake (-DNRN_ENABLE_CORENEURON=ON -DNRN_ENABLE_INTERVIEWS=OFF -DNMODL_SANITIZERS=undefinedundefined)	10m 12s
ubuntu-22.04 - cmake (-DNRN_ENABLE_CORENEURON=ON -DNRN_ENABLE_INTERVIEWS=OFF -DNMODL_SANITIZERS=addressaddress)	10m 46s
ubuntu-22.04 - cmake (-DNRN_ENABLE_CORENEURON=ON -DNRN_ENABLE_MPI=OFF -DCORENRN_ENABLE_OPENMP=OFF -DNRN_ENABLE_RX3D=OFFthread)	23m 41s
macOS-13 - cmake (-DNRN_ENABLE_CORENEURON=ON -DNRN_ENABLE_INTERVIEWS=OFF -DNRN_ENABLE_RX3D=OFF -DNMODL_SANITIZERS=addressaddress)	21m 36s
Final CI	1s
	3h 3m 3s

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