Update all jobs to avoid ubuntu-20.04 image removal

[rtibbles]

Summary

• Updates all ubuntu-20.04 images to ubuntu-latest.
• Puts all EOL Python jobs into docker containers with the appropriate Python installed
• Adds an environment variable to skip using the C extensions installed in the wheel
• Removes our C extension and no-c-extension tests in favour of doing a smoke test of the built whl file (with and without the above environment variable)
• Updates the content directory sanity check test to use a file path that is unwriteable even for root

References

Fixes the Kolibri part of learningequality/kolibri-ecosystem#41

Reviewer guidance

Does everything still pass?

Note that this changes the names of our Python tests, and deletes the C and no C extension tests, so some of our required tests now have different names - these will need to be updated after this is merged (with a required check bypass).

[github-actions]

Build Artifacts

Asset type	Download link
PEX file	kolibri-0.18.0a2.dev0_git.52.gaa346b9d.pex
Windows Installer (EXE)	kolibri-0.18.0a2.dev0+git.52.gaa346b9d-windows-setup-unsigned.exe
Debian Package	kolibri_0.18.0a2.dev0+git.52.gaa346b9d-0ubuntu1_all.deb
Mac Installer (DMG)	kolibri-0.18.0a2.dev0+git.52.gaa346b9d.dmg
Android Package (APK)	kolibri-0.18.0a2.dev0+git.52.gaa346b9d-0.1.4-debug.apk
Raspberry Pi Image	kolibri-pi-image-0.18.0a2.dev0+git.52.gaa346b9d.zip
TAR file	kolibri-0.18.0a2.dev0+git.52.gaa346b9d.tar.gz
WHL file	kolibri-0.18.0a2.dev0+git.52.gaa346b9d-py2.py3-none-any.whl