fix bake definition

[crazy-max]

Saw this after merging #103: https://github.com/tonistiigi/xx/actions/runs/5521258598/jobs/10069278303#step:7:120

  /usr/bin/docker buildx bake --file ./docker-bake.hcl --file /tmp/docker-actions-toolkit-681WDL/docker-metadata-action-bake.json --metadata-file /tmp/docker-build-push-RnQdwz/metadata-file --push base-all --print
  ERROR: failed to find target base-all
  Error: The process '/usr/bin/docker' failed with exit code 1

Looks like an oversight.

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TEST_BASE_TYPE was also not set in the workflow.