Knative Serving 0.4+ depends on golang 1.12 to be fully usable.

From pmorie:

• add a golang 1.12 image here: https://github.com/openshift/release/tree/master/projects/origin-release
• let's pull the binary from the go site for now; we can evolve to take the approach other images are using of doing the RPM install as 1.12 is packaged, or update as needed for a later rc in the interim
• let's build with 1.12 for now, i don't want to have to get into detail with users about how we built with 1.11, therefore some expected capability is missing

Acceptance Criteria:

• We can build images based on golang 1.12 in our CI (required for Knative 0.4)