[Tile-serving] [openstreetmap/mod_tile] Problems with CI pipeline (#256)

[ǝɹʇʇɐʃǝ◖ xıʃǝɟ]

We are facing issues with the CI pipeline, based on Github Actions. They fail with this error:

```
Error: `uses:` keyword is not currently supported.
```

We use `uses:` in our github actions definition. It worked before and is [part of the official documentation](https://docs.github.com/en/actions/reference/workflow-syntax-for-github-actions#jobsjob_idstepsuses). @hummeltech, by chance do you know what is going on?

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https://github.com/openstreetmap/mod_tile/issues/256
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