ci: use 20.04 on x86_64-gnu-nopt builder

This commit switches the x86_64-gnu-nopt builder to use Ubuntu 20.04,
which contains a more recent gdb version than Ubuntu 16.04 (newer gdb
versions fix a bug that Split DWARF can trigger, see
rust-lang#77177 for motivation). x86_64-gnu-nopt is chosen because
it runs compare modes, which is how Split DWARF testing is implemented
in rust-lang#77177.

Signed-off-by: David Wood <david@davidtw.co>

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

@@ -1,6 +1,7 @@
-FROM ubuntu:16.04
+FROM ubuntu:20.04
 
-RUN apt-get update && apt-get install -y --no-install-recommends \
+# Avoid interactive prompts while installing `tzdata` dependency with `DEBIAN_FRONTEND`.
+RUN apt-get update && DEBIAN_FRONTEND=noninteractive apt-get install -y --no-install-recommends \
   g++ \
   make \
   ninja-build \