updated the default paths such that both of the scripts can be run from the root repo

[jgunstone]

updated the default filepaths of the sort.py and check-duplicates.py scripts point at BDNS_Abbreviations_Register.csv file when the scripts are run from the repo root.

this simplifies the use of the scripts as you don't need to explicitly give the filepath.

i.e.

python tools/check-duplicates.py
python tools/sort.py

this doesn't affect the GH actions which explicity give the filepath.