[R] Nightly builds failing due to dataset test not being skipped on builds without datasets module

[thisisnic]

Describe the bug, including details regarding any error messages, version, and platform.

Nightly builds where datasets aren't installed are failing due to a recently-introduced test using datasets, e.g. https://dev.azure.com/ursacomputing/crossbow/_build/results?buildId=42831&view=logs&j=0da5d1d9-276d-5173-c4c4-9d4d4ed14fdb&t=d9b15392-e4ce-5e4c-0c8c-b69645229181

══ Failed tests ════════════════════════════════════════════════════════════════
── Error ('test-dplyr-query.R:745'): Can use nested field refs ─────────────────
Error: This build of the arrow package does not support Datasets
Backtrace:
    ▆
 1. ├─arrow:::expect_equal(...) at test-dplyr-query.R:745:2
 2. │ └─base::inherits(object, "ArrowObject") at tests/testthat/helper-expectation.R:34:2
 3. ├─... %>% collect()
 4. ├─dplyr::collect(.)
 5. ├─dplyr::filter(., nested > 7)
 6. ├─dplyr::mutate(., nested = df_col$a, times2 = df_col$a * 2)
 7. └─InMemoryDataset$create(.)
 8.   └─arrow:::stop_if_no_datasets()

[ FAIL 1 | WARN 0 | SKIP 117 | PASS 6415 ]
Error: Test failures
Execution halted

1 error ✖ | 0 warnings ✔ | 2 notes ✖
Error: R CMD check found ERRORs
Execution halted
1
##[error]Bash exited with code '1'.

Component(s)

R