Take leverage of variables into labels

[lvjp]

Description

To help the resolution of #403, when SOURCE_DATE_EPOCH is the commit date, since #471, we can now use the variable commit_date but it does not interpolate within the labels.

With this input :

      - uses: docker/metadata-action@v5.6.1
        with:
          labels: |
            org.opencontainers.image.created={{commit_date YYYY-MM-DDTHH:mm:ssZ}}

My test :

--- expected
+++actual
-org.opencontainers.image.created=2024-12-06T04:45:03+01:00
+org.opencontainers.image.newcreated={{commit_date 'YYYY-MM-DDTHH:mm:ssZ'}}