Arcyd: store full path of external report command

Handle external report commands more robustly by using their full path,
this means that if we later change directories in arcyd then we'll still
be able to use them. This is consistent with the
'--external-error-logger' behaviour.

Test Plan:

Just in case
$ ./precommit.sh

Check that json is still logged as expected
$ ./testbed/arcyd/test_shell.sh
    $ tail -f ../arcyd/savereport.txt
    .. logs as expected ..