MINOR: [R] fix no visible global function definition: left_join (apac…

…he#41542)

### Rationale for this change

Followup to apache#41350, fixes a check NOTE that caused.

### What changes are included in this PR?

`dplyr::` in two places.

### Are these changes tested?

Check will be clean.

### Are there any user-facing changes?

🙅

r/R/dplyr-mutate.R

@@ -84,12 +84,12 @@ mutate.arrow_dplyr_query <- function(.data,
     agg_query$aggregations <- mask$.aggregations
     agg_query <- collapse.arrow_dplyr_query(agg_query)
     if (length(grv)) {
-      out <- left_join(out, agg_query, by = grv)
+      out <- dplyr::left_join(out, agg_query, by = grv)
     } else {
       # If there are no group_by vars, add a scalar column to both and join on that
       agg_query$selected_columns[["..tempjoin"]] <- Expression$scalar(1L)
       out$selected_columns[["..tempjoin"]] <- Expression$scalar(1L)
-      out <- left_join(out, agg_query, by = "..tempjoin")
+      out <- dplyr::left_join(out, agg_query, by = "..tempjoin")
     }
   }