build: include environment variables for downloading bitcoin
build: include environment variables for downloading bitcoin

for parent modules to specify npm rebuild and install behavior

snogcel/bitcore-node-dash,kleetus/bitcore-node,zcoinrocks/bitcore-node,zcoinrocks/bitcore-node,braydonf/bitcore-node,braydonf/bitcore-node,snogcel/bitcore-node-dash,kleetus/bitcore-node