gh: Provide option to build `hub` without autoupdate

Use Go build tag to build with/without autoupdate. For Homebrew, we build with `./script/build noupdate` and let it handle the update. We could also build with `./script/build` which has autoupdate built-in.

/cc @mislav 
Is this how build tags work? This magic comment makes this file get included only if the tag is enabled?