cmd-build-with-buildah: add FCOS stream label manually

This is for coreos/fedora-coreos-tracker#1996.
I initially wanted to just add another `LABEL` to our Containerfile
and hook that up to another build arg, but the problem is that our
Containerfile is shared with RHCOS/SCOS and there's no way to make
`LABEL` directives conditional.

I opened coreos/rpm-ostree#5454 which will fix
this but for now to not block on this, let's just slap it on from the
cosa side. Once it's folded back into the build process proper, then we
can rever this. But the label not existing shouldn't really affect the
majority of developers anyway.

Author: jlebon

src/cmd-build-with-buildah

@@ -73,8 +73,10 @@ build_with_buildah() {
     initconfig="src/config.json"
     if [ -f "${initconfig}" ]; then
         variant="$(jq --raw-output '."coreos-assembler.config-variant"' "${initconfig}")"
+        manifest="src/config/manifest-${variant}.yaml"
         argsfile=build-args-${variant}.conf
     else
+        manifest="src/config/manifest.yaml"
         argsfile=build-args.conf
     fi
 
@@ -88,6 +90,14 @@ build_with_buildah() {
             --build-arg VERSION="${VERSION}" \
             -t oci-archive:"${tmp_oci_archive_path}"
 
+    # XXX: Temporary hack until we have https://github.com/coreos/rpm-ostree/pull/5454
+    # which would allow us to fold this back into the build process.
+    # shellcheck source=/dev/null
+    stream=$(yaml2json "$manifest" /dev/stdout | jq -r '.variables.stream')
+    if [ "${stream}" != null ]; then
+        set -- "$@" --label fedora-coreos.stream="$stream"
+    fi
+
     if [ -n "$DIRECT" ]; then
         # turn on layer caching in the direct case; it wouldn't hurt in the
         # supermin path, but it'd be a waste of space on the rootfs