fix(ci): fix inline expansion of date tag

[youtalk]

Description

This PR fixes the bug where the date tag is not expanded inline due to the lack of writing $ prefix.

https://github.com/youtalk/autoware/actions/runs/9886057164/job/27305134747#step:5:480

Run docker/metadata-action@v5
  with:
    images: ghcr.io/youtalk/autoware
    tags: {{date 'YYYYMMDD'}}
  latest
  latest-humble
    bake-target: docker-metadata-action-base
    flavor: latest=false
  suffix=-base

Tests performed

Not applicable.

Effects on system behavior

Not applicable.

Interface changes

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[youtalk]

I found this is correct https://github.com/docker/metadata-action?tab=readme-ov-file#date-format-tztimezone.