arrow

MINOR: [R] refactor arrow_mask to include aggregations list (#41414)

### Rationale for this change

Keeping the `..aggregations` list in parent.frame felt a little wrong.
As we're starting to use this in more places (like mutate in #41350, and
potentially more places), I wanted to try to improve this. I tried a
bunch of things before to put it somewhere better (like in the mask) but
failed. Finally I found one that worked.

### What changes are included in this PR?

Just a refactor

### Are these changes tested?

Existing tests pass.

### Are there any user-facing changes?

Nope.