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MINOR: [R] add back `dplyr::` to `left_join` calls (#43348)

Necessary for a clean check. These were inadvertently taken out in #41576 and don't actually change any code, just appeases the static checker that CRAN runs.

Authored-by: Jonathan Keane <jkeane@gmail.com>
Signed-off-by: Jonathan Keane <jkeane@gmail.com>