If the Pretested Configuration Plugin is configured to wipe out and re-clone your repository (as recommended) it will mean that the timestamps of all of your files will be reset to the time of the re-clone. When ccache is processing source files this is not normally a concern. For compliers it is a different matter. Ccache's default "compiler_check" strategy is to check the "mtime" of your compiler. If your compiler is contained within your repository the "mtime" will be constantly changing and thus you will never get ccache hits.
And you will be sad.
The solution is to change the "compiler_check" strategy to something more appropriate to your situation (e.g. "content"). Read the "compiler_check" section of the ccache manual for more information: https://ccache.samba.org/manual.html.