Install gcovr to Debian Buster amd64 image

Also sort "VENDOR-VERSION-ARCH" variables.
7 jobs for !72 with mnowak/add-gcovr in 14 minutes and 27 seconds (queued for 2 seconds)
detached
Status Job ID Name Coverage
  Images
passed #1046921
amd64 linux docker
debian:buster:amd64:docker

00:14:27

canceled #1046922
amd64 linux docker
debian:sid:amd64:docker

00:00:27

canceled #1046923
arm64 linux docker
debian:sid:arm64:docker

canceled #1046926
i386 linux docker
debian:sid:i386:docker

00:00:27

canceled #1046920
amd64 linux docker
debian:stretch:amd64:docker

00:00:30

canceled #1046925
amd64 linux docker
ubuntu:focal:amd64:docker

00:00:29

canceled #1046924
amd64 linux docker
ubuntu:xenial:amd64:docker

00:00:30