Makefile: Use go env GOPATH command to get GOPATH rather than env var GOPATH
Makefile: Use go env GOPATH command to get GOPATH rather than env var GOPATH

It is convention now to leave GOPATH unset, and if so defaults (via `go env` command) to ~/go.

So this makes things work on a system that doesn't have GOPATH env var set.

Signed-off-by: Thomas Parrott <6b778ce645fb0e3dde76d79eccad490955b1ae74@canonical.com>

stgraber/lxd,atxwebs/lxd,lxc/lxd,stgraber/lxd,lxc/lxd,stgraber/lxd,lxc/lxd,hallyn/lxd,stgraber/lxd,stgraber/lxd,hallyn/lxd,atxwebs/lxd,hallyn/lxd,lxc/lxd,atxwebs/lxd,stgraber/lxd,hallyn/lxd,lxc/lxd,hallyn/lxd,lxc/lxd,stgraber/lxd,atxwebs/lxd,hallyn/lxd,lxc/lxd