[ibd.sh] Store datadir and debug.log paths in variables

Summary:
This makes it easy to change the location of the datadir or debug.log without updating it in a bunch of places and risking
the chance of forgetting or typos.

Test Plan: IBD on TeamCity via D3453

Reviewers: #bitcoin_abc, deadalnix, Fabien

Reviewed By: #bitcoin_abc, deadalnix, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D3452