CCache cache not saved

[Flamefire]

Looks like the ccache action doesn't actually save the cache. This is from the post-step:

Cache hit occurred on the primary key ubuntu-20.04--gcc-11, not saving cache.

So it doesn't update the cache! O.o

We could use a dedicated ccache action like https://github.com/hendrikmuhs/ccache-action which handles the key-creation etc. for us or do it manually (see fix_ccache branch which I'm testing right now)

[Flamefire]

Closed (for now) by 699bb3b which takes the "manual" approach to not needing to trust another action

[Flamefire]

Note for library authors: the following change reuse the existing (old) cache if any:

          key: ${{matrix.os}}-${{matrix.container}}-${{matrix.compiler}}-${{github.sha}}
          restore-keys: |
            ${{matrix.os}}-${{matrix.container}}-${{matrix.compiler}}-
            ${{matrix.os}}-${{matrix.container}}-${{matrix.compiler}}

For new caches and after the transition only the first restore key is required