CI: Fix default kayobe base image when built on push

Inputs are not available to workflows triggered by a push. Apply a default to the base image.

(cherry picked from commit 359ee38)

Author: markgoddard

.github/workflows/stackhpc-build-kayobe-image.yml

@@ -92,7 +92,7 @@ jobs:
           build-args: |
              http_proxy=${{ inputs.http_proxy }}
              https_proxy=${{ inputs.https_proxy }}
-             BASE_IMAGE=${{ inputs.base_image }}
+             BASE_IMAGE=${{ inputs.base_image || 'rockylinux:9' }}
              KAYOBE_USER_UID=1001
              KAYOBE_USER_GID=1001
           push: true