Deduplicate environment variables in CI

[cameel]

While working on #13583 I initially made some simple mistakes when defining environment variables for CI jobs, and realized that we can do it better to make such mistakes less likely.

This PR is a simple refactor to make jobs extend the set of environment variables defined in base image definitions instead of redefining them.

Details

Currently, when we want an extra variable for a job, but still need the variables we defined for the base image, we repeat them like this:

defaults:
  - base_ubuntu_clang: &base_ubuntu_clang
      docker:
        - image: << pipeline.parameters.ubuntu-clang-docker-image >>
      environment:
        TERM: xterm
        CC: clang
        CXX: clang++
        MAKEFLAGS: -j 3

jobs:
  b_ubu_clang:
    <<: *base_ubuntu_clang
    environment:
      TERM: xterm
      CC: clang
      CXX: clang++
      MAKEFLAGS: -j 3
      CMAKE_OPTIONS: << parameters.cmake_options >>

This is error-prone and verbose. In many cases we don't even repeat variables a job does not use, to make the job definition shorter.

The PR replaces this convention with reuse of environment dicts via YAML anchors like this:

defaults:
  - base_ubuntu_clang: &base_ubuntu_clang
      docker:
        - image: << pipeline.parameters.ubuntu-clang-docker-image >>
      environment: &base_ubuntu_clang_env
        TERM: xterm
        CC: clang
        CXX: clang++
        MAKEFLAGS: -j 3

jobs:
  b_ubu_clang:
    <<: *base_ubuntu_clang
    environment:
      <<: *base_ubuntu_clang_env
      CMAKE_OPTIONS: << parameters.cmake_options >>

[r0qs]

LGTM just need a rebase :)

[cameel]

Rebased.