Use GOPATH variable if defined in installation from source

This makes the command work on systems with a custom GOPATH environment
variable set. If not defined, it will fallback to `$HOME/.go`.

content/getting-started/installing.md

@@ -79,7 +79,7 @@ The simplest way is to use [mage][] (a Make alternative for Go projects.)
 {{< code file="from-gh.sh" >}}
 go get github.com/magefile/mage
 go get -d github.com/gohugoio/hugo
-cd $HOME/go/src/github.com/gohugoio/hugo
+cd ${GOPATH:-$HOME/go}/src/github.com/gohugoio/hugo
 mage vendor
 mage install
 {{< /code >}}