Run CI on Ubuntu 20.04
I believe recent failures in CI (`AddressSanitizer::DEADLYSIGNAL`
repeating ad infinitum before a single test actually runs) are due to
using a Debian image with an ASAN bug that affects systems where ASLR
has a wide range (32 bits), combined with running on Ubuntu-latest which
is now setting that ASLR range in the kernel.
See https://github.com/llvm/llvm-project/commit/fb77ca05ffb4f8e666878f2f6718a9fb4d686839
Switching to 20.04 seems to solve the issue.
Fixes #305