dev/sg: add background package, move updates to background

Created by: bobheadxi

Closes https://github.com/sourcegraph/sourcegraph/issues/37852 by moving updates into the background so that the user isn't interrupted. In the future this will also allow us to add background uploads of sg analytics as well.

Test plan

go build -ldflags "-X main.BuildCommit=dev-1234" -o ./sg ./dev/sg

With some fake background jobs:

With just the update check job, -v and without -v: