ci: update x86_64-gnu and x86_64-gnu-debug to ubuntu:20.04

The former `ubuntu:19.10` reached EOL in July, 2020, whereas
`ubuntu:20.04` is an LTS release supported until 2025.

These are non-dist CI images, so the impact should be low.

src/ci/docker/host-x86_64/x86_64-gnu-debug/Dockerfile

-FROM ubuntu:19.10
+FROM ubuntu:20.04
 
+ARG DEBIAN_FRONTEND=noninteractive
 RUN apt-get update && apt-get install -y --no-install-recommends \
   g++ \
   make \

src/ci/docker/host-x86_64/x86_64-gnu/Dockerfile

-FROM ubuntu:19.10
+FROM ubuntu:20.04
 
+ARG DEBIAN_FRONTEND=noninteractive
 RUN apt-get update && apt-get install -y --no-install-recommends \
   g++ \
   make \