[CI] Use ccache only for the pacxx-build step.
The other steps use a different compiler anyway.
Also, follow the style of pacxx-ci and pacxx-docker in defining a &ccache anchor and leaving the global before_script and after_script alone.
The other steps use a different compiler anyway.
Also, follow the style of pacxx-ci and pacxx-docker in defining a &ccache anchor and leaving the global before_script and after_script alone.