fix: spark connect client Harbor credentials (#1092)

Author: dervoeti

.github/workflows/reusable_build_image.yaml

@@ -93,7 +93,7 @@ jobs:
         uses: stackabletech/actions/publish-index-manifest@2d3d7ddad981ae09901d45a0f6bf30c2658b1b78 # 0.7.0
         with:
           image-registry-uri: oci.stackable.tech
-          image-registry-username: robot$sdp+github-action-build
+          image-registry-username: robot$${{ inputs.registry-namespace }}+github-action-build
           image-registry-password: ${{ secrets.harbor-robot-secret }}
           image-repository: ${{ inputs.registry-namespace }}/${{ inputs.product-name }}
           image-index-manifest-tag: ${{ matrix.versions }}-stackable${{ inputs.sdp-version }}