[CI] Fix wrong path to bitcoind binary on build-ibd-*

Summary:
The ibd.sh and bitcoind-exit-on-log.sh scripts were relying on the
default build directory to find the bitcoind binary to run. This is
causing failure since D6551.

Test Plan: Run the ci build-ibd* build plans.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6568