hack: pin the os release base url, channel and build name

Three environment variables to be used when building the pinned release binary:
    RHCOS_BASE_URL: points to the base url from where to pick the image from. Defaults to "https://releases-rhcos.svc.ci.openshift.org/storage/releases".
    RHCOS_DEFAULT_CHANNEL: points to the channel where the image is available. Defaults to "maipo"
    RHCOS_BUILD_NAME: a string representing the build name. If empty, then the latest one will be picked up.

hack/build.sh

@@ -32,6 +32,18 @@ release)
 	then
 		LDFLAGS="${LDFLAGS} -X github.com/openshift/installer/pkg/asset/ignition/bootstrap.defaultReleaseImage=${RELEASE_IMAGE}"
 	fi
+	if test -n "${RHCOS_DEFAULT_CHANNEL}"
+	then
+		LDFLAGS="${LDFLAGS} -X github.com/openshift/installer/pkg/rhcos.DefaultChannel=${RHCOS_DEFAULT_CHANNEL}"
+	fi
+	if test -n "${RHCOS_BASE_URL}"
+	then
+		LDFLAGS="${LDFLAGS} -X github.com/openshift/installer/pkg/rhcos.baseURL=${RHCOS_BASE_URL}"
+	fi
+	if test -n "${RHCOS_BUILD_NAME}"
+	then
+		LDFLAGS="${LDFLAGS} -X github.com/openshift/installer/pkg/rhcos.buildName=${RHCOS_BUILD_NAME}"
+	fi
 	if test "${SKIP_GENERATION}" != y
 	then
 		go generate ./data

pkg/rhcos/builds.go

@@ -11,10 +11,14 @@ import (
 	"github.com/sirupsen/logrus"
 )
 
-const (
+var (
 	// DefaultChannel is the default RHCOS channel for the cluster.
 	DefaultChannel = "maipo"
 
+	// buildName is the name of the build in the channel that will be picked up
+	// empty string means the first one in the build list (latest) will be used
+	buildName = ""
+
 	baseURL = "https://releases-rhcos.svc.ci.openshift.org/storage/releases"
 )
 
@@ -104,5 +108,14 @@ func fetchLatestBuild(ctx context.Context, channel string) (string, error) {
 		return "", errors.Errorf("no builds found")
 	}
 
+	if buildName != "" {
+		for _, build := range builds.Builds {
+			if build == buildName {
+				return buildName, nil
+			}
+		}
+		return "", errors.Errorf("no build with name '%s' found at '%s'. Available builds are: %v", buildName, url, builds.Builds)
+	}
+
 	return builds.Builds[0], nil
 }