Add most recent Ubuntu LTS (20.04) to our automated testing

[daverodgman]

Add support for most recent Ubuntu LTS - currently this is 20.04 (and the next will be 22.04). We could consider adding e.g. 21.04 (or both) but probably should always aim to have the most recent LTS in our testing.

[gilles-peskine-arm]

Clang's UBsan in Ubuntu 20.04 catches some mistakes that we tend to make, and that nothing on the current CI with older Clang catches. Specifically, more than once, I've seen that an Asan+UBsan build passed on the CI but fails on my Ubuntu 20.04 machine because some code does p + x with p==NULL and x==0, which is harmless in practice but technical undefined behavior.

[minosgalanakis]

As part of the work needed for PR31 I have to enable testing for 20.04.

Unfortunately it did not pass all tests, and some dependencies have been identified. The linked PR is not to be merged until the tasks identified at is are addressed.