Re-add the Go step via go-global-update (#823)

* Re-add the Go step via go-global-update

go-global-update (https://github.com/Gelio/go-global-update) is a small
tool to update all executables installed in a user's GOBIN, effectively
providing a `go get -u all` replacement for Go 1.16+.

Since it appears that Go will not be receiving a built-in way to do this
in the near future, this could be a good option in the meantime.

* Skip silently if go-global-update is not present

src/config.rs

@@ -88,6 +88,7 @@ pub enum Step {
     Gcloud,
     Gem,
     GitRepos,
+    Go,
     Haxelib,
     GnomeShellExtensions,
     HomeManager,