openshift/os: temporarily work around missing /go directory in cosa

Effectively carry coreos/coreos-assembler#3957
until it lands and cosa gets rebuilt.

ci-operator/config/openshift/os/openshift-os-master.yaml

@@ -1,8 +1,10 @@
 build_root:
-  image_stream_tag:
-    name: coreos-assembler
-    namespace: coreos
-    tag: latest
+  project_image:
+    dockerfile_literal: |
+      FROM quay.io/coreos-assembler/coreos-assembler:latest
+      USER root
+      RUN mkdir -p /go && chmod 777 /go
+      USER builder
 images:
 - dockerfile_literal: |
     FROM overridden