Consider not downloading anything during install

[julianfairfax]

This would make offline builds (like inside a Flatpak) possible. I don't see any downsides either, since every update to sqlcipher.tar.gz requires an update to https://github.com/signalapp/better-sqlite3/blob/better-sqlcipher/deps/download.js anyway (so you could just update the file instead of the script).