Add bootstrap script for sg

Created by: mrnugget

This allows users to run the following:

curl --proto '=https' --tlsv1.2 -sSf https://raw.githubusercontent.com/sourcegraph/sourcegraph/main/dev/sg/bootstrap.sh | sh

Or, before it's merged:

curl --proto '=https' --tlsv1.2 -sSf https://raw.githubusercontent.com/sourcegraph/sourcegraph/bc92c5fb27b8455ff53a869ea9a6e496a3121ec7/dev/sg/bootstrap.sh | sh

It downloads sg's latest release and starts the new sg install command that copies the newly-downloaded binary into a location and adds that location to the $PATH.