hack/build: Pin to quay.io/openshift-release-dev/ocp-release:4.0.0-0.8

Clayton pushed
registry.svc.ci.openshift.org/ocp/release:4.0.0-0.nightly-2019-03-23-183709
to quay.io/openshift-release-dev/ocp-release:4.0.0-0.8.

Renaming OPENSHIFT_INSTALL_RELEASE_IMAGE_OVERRIDE gets us CI testing
of the pinned release despite openshift/release@60007df2 (Use
RELEASE_IMAGE_LATEST for CVO payload, 2018-10-03,
openshift/release#1793).

I'd initially expected to export the pinning environment variables in
release.sh, but I've put them in build.sh here because our continuous
integration tests use build.sh directly and don't go through
release.sh.

The release-image change-log was generated using the generator from
[1] and:

  $ oc adm release info --changelog ~/.local/lib/go/src --changes-from quay.io/openshift-release-dev/ocp-release:4.0.0-0.7 quay.io/openshift-release-dev/ocp-release:4.0.0-0.8

[1]: openshift/origin#22030