arrow

GH-43627: [R] Fix summarize() performance regression (pushdown) (#43649)

### Rationale for this change

See https://github.com/apache/arrow/issues/43627#issuecomment-2284259559

### What changes are included in this PR?

An extra `dplyr::select()`

### Are these changes tested?

Conbench should show that the performance is much better

### Are there any user-facing changes?

Not slow
* GitHub Issue: #43627