refactor: update arx_forecaster predictor arg

[dshemetov]

Checklist

Please:

• [x] Make sure this PR is against "dev", not "main".
• [x] Request a review from one of the current epipredict main reviewers: dajmcdon.
• [x] Makes sure to bump the version number in DESCRIPTION and NEWS.md. Always increment the patch version number (the third number), unless you are making a release PR from dev to main, in which case increment the minor version number (the second number).
• [x] Describe changes made in NEWS.md, making sure breaking changes (backwards-incompatible changes to the documented interface) are noted. Collect the changes under the next release number (e.g. if you are on 0.7.2, then write your changes under the 0.8 heading).

Change explanations for reviewer

Update the predictor default to the value of the outcome arg in arx_forecaster.

Magic GitHub syntax to mark associated Issue(s) as resolved when this is merged into the default branch

• Resolves #298

[dshemetov]

Made the requested updates and force-pushed.

---

I'm getting the error locally

Error (test-target_date_bug.R:57:3): arx_classifier determines target_date where forecast_date exists
Error in `check_installs(object)`: This engine requires some package installs: 'xgboost'

which comes from the line

# test-target_date_bug.R, line 59
    trainer = parsnip::boost_tree(mode = "classification", trees = 5),

I guess xgboost is present on your machine and on CI, but I'm missing it, not sure how. Should we add it to Suggests or change the example to minimize dependencies? @dajmcdon

[dajmcdon]

I suspect it's just a problem local to your machine. {xgboost} is in Suggests already, so it will be installed in CI.