Custom path for index download in installation sample code

[Soham Dan]

Hi, when I run
python -m pyserini.search.lucene \ --topics msmarco-passage-dev-subset \ --index msmarco-v1-passage \ --output run.msmarco-passage.txt \ --output-format msmarco \ --bm25
it downloads the 2 GB indices to my home directory.
How do I set a custom download location for the large index files?