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ARROW-9743: [R] Sanitize paths in open_dataset

Fixes problem of path expansion (`open_dataset("~/path")` now works). Unfortunately, tests can only write to tmp, so I can't add a test that confirms this, but I did verify locally.

Closes #7960 from nealrichardson/r-dataset-path

Authored-by: Neal Richardson <neal.p.richardson@gmail.com>
Signed-off-by: Neal Richardson <neal.p.richardson@gmail.com>