openshift/os: remove useless build_root image

We don't have a `binary_build_commands` nor use the `pipeline:root`
image so specifying a `build_root` is useless.

Author: jlebon

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

@@ -3,11 +3,6 @@ base_images:
     name: coreos-assembler
     namespace: coreos
     tag: latest
-build_root:
-  image_stream_tag:
-    name: fcos-buildroot
-    namespace: coreos
-    tag: testing-devel
 images:
 - dockerfile_literal: |
     # COSA + source code
@@ -113,9 +108,9 @@ resources:
       memory: 3Gi
 tests:
 - as: validate
-  commands: ci/prow-entrypoint.sh validate
+  commands: cd /src && ci/prow-entrypoint.sh validate
   container:
-    from: src
+    from: build-image
   skip_if_only_changed: ^docs/|\.md$|^(?:.*/)?(?:\.gitignore|OWNERS|PROJECT|LICENSE)$
 - as: rhcos-9-build-test-qemu
   cluster: build02