Add build tag `noupdate` to disable autoupdate
Add build tag `noupdate` to disable autoupdate

We take a global environment variable `GH_AUTOUPDATE` to control the on/off of autoupdate. We override this setting for the `noupdate` build. See https://github.com/jingweno/gh/blob/master/commands/updater.go#L251.

jingweno/gh,jingweno/gh,beni55/gh,jingweno/gh,beni55/gh,beni55/gh