chg: [GitHub action] install the python-cti-stix2 from the local submodule

.github/workflows/main.yml

@@ -194,6 +194,9 @@ jobs:
               sudo -E su $USER -c 'app/Console/cake Admin setSetting "MISP.python_bin" "$GITHUB_WORKSPACE/venv/bin/python"'
               . ./venv/bin/activate
               export PYTHONPATH=$PYTHONPATH:./app/files/scripts
+              pushd ./app/files/scripts/cti-python-stix2
+              pip install .
+              popd
               pushd PyMISP
               pip install .[fileobjects,email]
               popd