fix set-output deprecation messages in apko-publish

- kudos to @andros21 for the fix in chainguard-images#127
- fixes chainguard-images#140

```
The `set-output` command is deprecated and will be disabled soon. Please upgrade to using Environment Files. For more information see: https://github.blog/changelog/2022-10-11-github-actions-deprecating-save-state-and-set-output-commands/
```

Co-authored-by: andros21 <[email protected]>

apko-publish/action.yaml

@@ -175,6 +175,6 @@ runs:
  ${{ inputs.debug && '--log-level debug' }} \
  --image-refs="${{ inputs.image_refs }}" ${{ inputs.config }} ${{ inputs.tag }} $keys $repos $packages $archs $build_options $sbomPath | tee ${DIGEST_FILE}
  echo EXIT CODE: $?
- echo ::set-output name=digest::$(cat ${DIGEST_FILE})
+ echo "digest=$(cat ${DIGEST_FILE})" >> $GITHUB_OUTPUT
  EOF
  )"