Here is what stable package contains (filtered to dist-packages):
$ dpkg --contents /var/cache/apt/archives/kicad_5.1.8-202011242336+db9833491~88~ubuntu18.04.1_amd64.deb | grep dist
drwxr-xr-x root/root 0 2020-11-24 15:36 ./usr/lib/python3/dist-packages/
-rw-r--r-- root/root 22284952 2020-11-24 15:36 ./usr/lib/python3/dist-packages/_pcbnew.so
-rw-r--r-- root/root 787055 2020-11-24 15:36 ./usr/lib/python3/dist-packages/pcbnew.py
Same with nightly
$ dpkg --contents /var/cache/apt/archives/kicad-nightly_202011261018+09587ef4f~106~ubuntu18.04.1_amd64.deb | grep dist
drwxr-xr-x root/root 0 2020-11-26 02:18 ./usr/lib/kicad-nightly/lib/python3/dist-packages/
-rw-r--r-- root/root 30797728 2020-11-26 02:18 ./usr/lib/kicad-nightly/lib/python3/dist-packages/_pcbnew.so
-rw-r--r-- root/root 834627 2020-11-26 02:18 ./usr/lib/kicad-nightly/lib/python3/dist-packages/pcbnew.py
Looks like you will have to manually add /usr/lib/kicad-nightly/lib/python3/dist-packages to PYTHON_PATH and LD_LIBRARY_PATH to load nightly pcbnew python module.