Add instructions on how to theoratically build without Flatpak

3 jobs for master in 3 minutes and 28 seconds (queued for 3 seconds)
Name Stage Failure
failed
pytest Test
rootdir: /builds/nicoalt/briar-gtk, inifile: pytest.ini
plugins: mock-1.12.1, requests-mock-1.7.0, cov-2.8.1
collected 22 items

tests/briar/api/test_constants.py .... [ 18%]
tests/briar/api/models/test_contacts.py ... [ 31%]
tests/briar/api/models/test_private_chat.py .. [ 40%]
tests/briar/gtk/test_application.py .....tools/tests/test-pytest.sh: line 6: 4404 Trace/breakpoint trap (core dumped) PYTHONPATH=src pytest --cov=src tests/
ERROR: Job failed: exit code 1