Update test-various to Ubuntu 20.04

The test command-setgroups.rs is adjusted to skip on musl, where
`sysconf(_SC_NGROUPS_MAX)` always returns a dummy value of 32,
even though the actual value is 65536. I'm not sure why this becomes
relevant only now though, as this was apparently the case since
kernel 2.6.4.

src/ci/docker/host-x86_64/test-various/Dockerfile

@@ -1,6 +1,6 @@
-FROM ubuntu:18.04
+FROM ubuntu:20.04
 
-RUN apt-get update && apt-get install -y --no-install-recommends \
+RUN apt-get update && DEBIAN_FRONTEND=noninteractive apt-get install -y --no-install-recommends \
   g++ \
   make \
   ninja-build \

src/test/ui/command/command-setgroups.rs

@@ -3,6 +3,7 @@
 // ignore-cloudabi
 // ignore-emscripten
 // ignore-sgx
+// ignore-musl - returns dummy result for _SC_NGROUPS_MAX
 
 #![feature(rustc_private)]
 #![feature(setgroups)]