fix(cvmfs): fetch checksums.sha512 along with the pilot files from s3

Dirac pilots trying to fetch pilot files from /cvmfs would not run because checksums.sha512 is not present.

https://gitlab.cern.ch/lhcb-core/tasks/lbtaskrun/-/blob/master/src/lbtaskrun/cvmfs/init.py?ref_type=heads#L631

Here is the typical error we get from the pilots:

URLError: <urlopen error [Errno 2] No such file or directory: '/cvmfs/lhcb.cern.ch/lhcbdirac/pilot/checksums.sha512'>