[electrum] fix get_merkle_root when Electrum ABC is not installed

Summary: The script cannot import electrumabc if it is not in the python PATH. We could install ElectrumABC as a package with pip install . in update-electrum-checkpoints.sh, but it is simpler to just hack sys.path to make sure the script always work with the latest version of the source code.

Test Plan:

pip uninstall ElectrumABC
cd ./contrib/source-control-tools
./patch-recipes/update-electrum-checkpoints.sh

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15355