OKD ARM scheduled builds fail due to invalid inputs

[ggiguash]

Some of the inputs end up empty when a scheduled job runs.
See this run for example.

Note the USHIFT_BRANCH and OKD_RELEASE_IMAGE arguments. They should be workflow_dispatch defaults, but it does not seem to work.

Building the MicroShift builder image
sudo podman build \
        -t "microshift-okd-builder" \
        --ulimit nofile=524288:524288 \
        --build-arg USHIFT_BRANCH="" \
        --build-arg OKD_VERSION_TAG="4.21.0-okd-scos.ec.6" \
        --build-arg OKD_RELEASE_IMAGE="/okd-release-arm64" \
        -f packaging/microshift-builder.Containerfile .