cmd/upspin-deploy-gcp: use go/build package to locate GOPATH
cmd/upspin-deploy-gcp: use go/build package to locate GOPATH

The GOPATH environment variable will now default to $HOME/go if it is
unset. Use the GOPATH from the go/build package's default environment to
inherit this property.

Change-Id: I17863461d1a3fc7dcefee3f04738df7e60dd9b96
Reviewed-on: https://upspin-review.googlesource.com/10264
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

upspin/gcp,upspin/gcp