Merging new commits to master causes numerous CI cache misses

[lisanna-dettwyler]

Building HEAD of master takes a long time when there are significant cache misses. This mostly affects macOS and docker image building. When a change is merged that causes cache misses in master, these misses also arise in subsequent rebuilds of pull requests even if they haven't been rebased/updated since. Some things that can be done about this:

• Additionally use the git merge-base as a cache key, which remains fixed if the pull request remains fixed:

  - id: merge-base
    shell: bash
    run: |
      MERGE_BASE=$(git merge-base ${{ github.head_ref }} ${{ github.base_ref }})
      echo "::set-output name=ref::$MERGE_BASE"
  - uses: actions/cache@v2
    key: ${{ github.head_ref }}
    restore-keys: |
      ${{ steps.merge-base.outputs.ref }}
      ${{ github.base_ref }})

• Generate build cache for master faster (on: push: master), i.e., significantly reign-in compile times