build: require browserfs clone before npm install
build: require browserfs clone before npm install

Without this, the initial `make` command after cloning a fresh repo
might fail.

Addresses part of #2

flat1101/browsix,b1101/browsix,flat1101/browsix,b1101/browsix,flat1101/browsix,b1101/browsix,flat1101/browsix,b1101/browsix