Similar to #510 for macOS 10.15, GitHub Actions has begun the deprecation of the Ubuntu 18.04 Actions runner, effective today. As per actions/runner-images#6002:

To raise awareness of the upcoming removal, we will temporarily fail jobs using Ubuntu 18.04. Builds that are scheduled to run during the brownout periods will fail.

To avoid further setbacks caused by arbitrarily failing CI -- and because Ubuntu 18.04 is nearing end-of-life anyway -- the Gridsync project should remove Ubuntu 18.04 CI jobs and/or drop support for Ubuntu 18.04 entirely.