`check_title_template` is supported internally, but not exposed via the
action interface. This commit exposes it and adds an example usage to
the `build` job.

Relates to:
7a62880

The current template var format `${{...}}` clashes with that used by
GitHub Actions, which makes usage unwieldy. Currently users need to
escape the `${{FILE_NAME}}` template in order to avoid it being
interpreted as a GHA template, so usage looks like:
```yaml
check_title_template: ${{ '${{FILE_NAME}}' }}
```

This commit changes the template format to `{{..}}`, so usage looks
like:
```yaml
check_title_template: '{{FILE_NAME}}'
```

This is a non-breaking change, as currently the input which makes use of
template vars isn't exposed via the action.yml file.