scripts: make glide-update run from $GOPATH
scripts: make glide-update run from $GOPATH

Glide should run inside $GOPATH.

Inspired by https://github.com/coreos/rkt/pull/2885.

coreos/fleet,dongsupark/fleet,dalbani/fleet,dalbani/fleet,dalbani/fleet,dongsupark/fleet,jonboulle/fleet,giantswarm/fleet,dongsupark/fleet,jonboulle/fleet,giantswarm/fleet,endocode/fleet,giantswarm/fleet,coreos/fleet,jonboulle/fleet,endocode/fleet,coreos/fleet,endocode/fleet