test rebuilds for focal and groovy

[Matthias Klose]

Done as part of a comparative test rebuild, there are two test rebuilds (main
only) for focal and groovy, with both the release and the updates pocket
enabled.  Results can be found at

https://people.canonical.com/~doko/ftbfs-report/test-rebuild-20210602-focal-focal.html
https://people.canonical.com/~doko/ftbfs-report/test-rebuild-20210601-groovy-groovy.html

The last test rebuild for focal from August 2020:
https://people.canonical.com/~doko/ftbfs-report/test-rebuild-20200810-focal-focal.html

Matthias