debuginfod: update mtime of interval_path as early as possible

debuginfod: update mtime of interval_path as early as possible

Call utime on interval_path file as soon as the thread is committed to
cleanup the cache files. This will prevent other threads trying to
also commit to cleaning the cache files. Having multiple threads try
to clean the cache simultaniously doesn't improve cleanup speed
because the threads will try to delete the files in the same order.

Signed-off-by: Mark Wielaard <mark@klomp.org>