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 
I've changed the build script to build with `noupdate` for now: https://github.com/github/hub/blob/cc816c9df8a188a833f82cc25c58148e4ac87543/script/build#L22-L23