fix: Use a cleanup stamp instead of directory timestamp for cleanup

fix: Use a cleanup stamp instead of directory timestamp for cleanup

In order to know when to clean up the temporary directory, ccache checks
mtime of $CCACHE_DIR and updates it to "now" on cleanup. This doesn't
work well when the configuration file is modified often since that also
updates the same mtime.

Fix this by using a separate cleanup stamp file instead.

(cherry picked from commit ec27506da3a03e0f5bf5f80494174fcdd9a43b0e)